CNN's Ryan Chilcote, who is nearby, said he could not tell if the explosion came from inside or outside the building, or if the blast was initiated by the police or the hostage-takers.
Police said the gunmen inside the theatre identified themselves as members of the 29th Division of the Chechen army. Officials say they are demanding an end to the war in Chechnya.
The hostage-takers, numbering between 40 and 50, have threatened to kill some of the hostages and have told police they have mined portions of the building -- The Palace of Culture of the Podshipnikov Zavod.
A pro-rebel Web site said the Russians had seven days to begin the withdrawal or the theatre and hostages would be blown up.
They have also asked for the Russian-installed head of the Chechen government to talk to them.
A short time after the blast was heard, German and Austrian Embassy officials arrived at the scene, where they were told they could win the release of Western hostages.
Citizens of Germany, Austria, The Netherlands and Britain as well as one American, are among those being held. But the Westerners were not immediately released.
Late on Thursday morning, the hostage-takers allowed fresh water into the theatre -- the first water taken inside in about 14 hours. They have also requested that a Russian TV crew be allowed to enter the building to shoot videotape of the scene, officials say.
In addition, the head of the Red Cross in Moscow was invited by security officials to play a possible mediation roll in the standoff.
Valeri Girbakin, a spokesman for Moscow police, said the armed contingent included an unspecified number of women of "non-Slavic" nationalities, some with explosives strapped to their bodies.
"They are demanding to resolve the situation in the Chechen Republic, specifically pulling out Russian troops," he said.
Hostages who have managed to telephone Russian radio stations on cell phones have said the gunmen have explosives, have mined parts of the building, and have threatening to kill 10 hostages for every hostage-taker wounded by police.
There was no immediate explanation for the blast heard on Thursday morning, and no reports to suggest security forces had begun to storm the building.
Police accounts varied widely, detailing anywhere from 400 to 700 hostages.
Chilcote said the authorities say 711 tickets were sold for the performance of "Nord-Ost" -- a popular Russian romantic-comedy -- and that the production contained about 100 actors and backstage staff.
He said of the 150 hostages thought to have been allowed to leave, 41 had been confirmed as freed or escaped, including a pregnant woman who was freed a few hours after the siege began.
Witnesses said the group stormed the theatre at about 9:05 p.m. (1700 GMT) on Wednesday night, fired several shots into the air and refused to let anyone leave the theatre.
"At the beginning of the second act, people in camouflage uniforms, they ran up on stage and they started firing from Kalashnikovs," said a woman who escaped from the theater.
"I didn't hear any political demands. There was a Georgian woman. They let her go. She said there was a lot of blood.
"It didn't seem like they were shooting people, it seemed like they were beating people. She said there was a lot of blood in the corridors."
Girbakin said the hostage-takers released some children, citizens of Georgia, foreigners and Muslims during the early stage of the drama.
One official, Gennady Gutkov told NTV television that the security forces would not storm the hall unless the rebels began killing hostages:
He said: "Storming of the building will not be carried out at the initiative of the Russian side if the terrorists do not undertake actions to kill large numbers of hostages."
Some 711 people had purchased tickets for the show
As dawn began to break over Moscow on Thursday the tense standoff continued. (Mood in Moscow)
Russia's Federal Security Service, the elite "Alpha" teams -- the equivalent of the former KGB -- are at the scene.
The theatre was bathed in lights and surrounded by security forces as negotiations continued for the release of the remaining hostages. (Images)
Russian President Vladimir Putin has cancelled a planned visit to Germany and Portugal Thursday to monitor the situation.
Ahmad Kadirov, head of the governing Chechen administration, called the incident a tragedy and said he was receiving calls from Chechens in Moscow who are scared about possible reprisals against them.
He said: "Those are terrorist bandits and I'm sure that one can't resolve problems by starting the war in another area.
"How could they get there? That's a question that interests me."
Russian forces left Chechnya in 1996 after a disastrous two-year war, but they returned in 1999 after rebels raided a neighbouring region. (Background)
The Russian authorities blamed rebels for a series of bombings in Russia that killed more than 300 people.
Police said the gunmen inside the theatre identified themselves as members of the 29th Division of the Chechen army. Officials say they are demanding an end to the war in Chechnya.
The hostage-takers, numbering between 40 and 50, have threatened to kill some of the hostages and have told police they have mined portions of the building -- The Palace of Culture of the Podshipnikov Zavod.
A pro-rebel Web site said the Russians had seven days to begin the withdrawal or the theatre and hostages would be blown up.
They have also asked for the Russian-installed head of the Chechen government to talk to them.
A short time after the blast was heard, German and Austrian Embassy officials arrived at the scene, where they were told they could win the release of Western hostages.
Citizens of Germany, Austria, The Netherlands and Britain as well as one American, are among those being held. But the Westerners were not immediately released.
Late on Thursday morning, the hostage-takers allowed fresh water into the theatre -- the first water taken inside in about 14 hours. They have also requested that a Russian TV crew be allowed to enter the building to shoot videotape of the scene, officials say.
In addition, the head of the Red Cross in Moscow was invited by security officials to play a possible mediation roll in the standoff.
Valeri Girbakin, a spokesman for Moscow police, said the armed contingent included an unspecified number of women of "non-Slavic" nationalities, some with explosives strapped to their bodies.
"They are demanding to resolve the situation in the Chechen Republic, specifically pulling out Russian troops," he said.
Hostages who have managed to telephone Russian radio stations on cell phones have said the gunmen have explosives, have mined parts of the building, and have threatening to kill 10 hostages for every hostage-taker wounded by police.
There was no immediate explanation for the blast heard on Thursday morning, and no reports to suggest security forces had begun to storm the building.
Police accounts varied widely, detailing anywhere from 400 to 700 hostages.
Chilcote said the authorities say 711 tickets were sold for the performance of "Nord-Ost" -- a popular Russian romantic-comedy -- and that the production contained about 100 actors and backstage staff.
He said of the 150 hostages thought to have been allowed to leave, 41 had been confirmed as freed or escaped, including a pregnant woman who was freed a few hours after the siege began.
Witnesses said the group stormed the theatre at about 9:05 p.m. (1700 GMT) on Wednesday night, fired several shots into the air and refused to let anyone leave the theatre.
"At the beginning of the second act, people in camouflage uniforms, they ran up on stage and they started firing from Kalashnikovs," said a woman who escaped from the theater.
"I didn't hear any political demands. There was a Georgian woman. They let her go. She said there was a lot of blood.
"It didn't seem like they were shooting people, it seemed like they were beating people. She said there was a lot of blood in the corridors."
Girbakin said the hostage-takers released some children, citizens of Georgia, foreigners and Muslims during the early stage of the drama.
One official, Gennady Gutkov told NTV television that the security forces would not storm the hall unless the rebels began killing hostages:
He said: "Storming of the building will not be carried out at the initiative of the Russian side if the terrorists do not undertake actions to kill large numbers of hostages."
Some 711 people had purchased tickets for the show
As dawn began to break over Moscow on Thursday the tense standoff continued. (Mood in Moscow)
Russia's Federal Security Service, the elite "Alpha" teams -- the equivalent of the former KGB -- are at the scene.
The theatre was bathed in lights and surrounded by security forces as negotiations continued for the release of the remaining hostages. (Images)
Russian President Vladimir Putin has cancelled a planned visit to Germany and Portugal Thursday to monitor the situation.
Ahmad Kadirov, head of the governing Chechen administration, called the incident a tragedy and said he was receiving calls from Chechens in Moscow who are scared about possible reprisals against them.
He said: "Those are terrorist bandits and I'm sure that one can't resolve problems by starting the war in another area.
"How could they get there? That's a question that interests me."
Russian forces left Chechnya in 1996 after a disastrous two-year war, but they returned in 1999 after rebels raided a neighbouring region. (Background)
The Russian authorities blamed rebels for a series of bombings in Russia that killed more than 300 people.
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