Soldiers and police have baton-charged Burmese protesters who tried to stage a further day of marches in Rangoon.
Security forces sealed off five monasteries that were focal points of previous mass marches, in a bid to prevent further demonstrations.
Official media said nine people were killed on Thursday as troops fired tear gas and bullets to clear large crowds of protesters off Rangoon's streets.
British and Australian ambassadors in Burma say the toll was probably higher.
"Observers say the death rate could be many multiples of that number, and I certainly wouldn't disagree with that," British ambassador Mark Canning told the BBC.
The security presence in Rangoon on Friday was the heaviest yet, says the BBC's South East Asia correspondent, Jonathan Head.
Troops sealed off the key religious sites in Rangoon, including the Shwedagon and Sule pagodas - the focal point of some of the larger protests earlier in the week.
All the main roads into central Rangoon have been blocked.
Information from Burma has become increasingly patchy. Internet access has been cut in Rangoon and is only partially available elsewhere.
Burmese sources told the BBC that international mobile phone signals have been interrupted and soldiers are searching people for cameras and mobile phones.
Dissidents have been using the internet to get pictures and video of the protests and the military crackdown to international news outlets - who then fed them back into Burma via the internet and satellite TV.
But eyewitnesses managed to contact the BBC on Friday to say that the government was sending bus-loads of vigilantes into the main city to attack the demonstrators.
They said a temporary holding pen had been set up at an old race course for the huge numbers of people detained in recent days.
The atmosphere is said to be extremely tense and there is a palpable sense of fear on the streets.
Correspondents say Rangoon looked like a city locked down on Friday morning but at about 1300 local time (0700GMT), small groups of protesters began gathering.
Some were immediately rounded up. Within minutes, shots could be heard - but it was not clear if they were being fired into the crowds or overhead in warning. A witness told the BBC that a number of people were killed in Friday's violence.
Loudspeaker trucks have been criss-crossing the city, warning people not to protect anyone fleeing arrest.
An overnight curfew for the South Okkalapa district, the scene of Thursday's violent encounters between soldiers and protesters, was also announced.
According to the Burmese officials accounts, a Japanese video journalist and eight protesters were killed on Thursday. One man was reported killed on Wednesday.
Japan is demanding a full explanation from Burma over the death of Kenji Nagai, an employee of the Tokyo-based news agency, APF News.
But Australian Ambassador Bob Davis told his country's ABC radio that the death toll might be "several multiples of the 10 acknowledged by the authorities".
He said witnesses had told embassy staff they had seen "significantly more than that number of dead being removed from the scene of the demonstrations" in Rangoon.
In addition to the dead, 11 demonstrators and 31 soldiers were hurt, according to the official account.
Authorities are trying to stamp out the largest uprising in two decades, led by Buddhist monks whose numbers on the streets appear to have dwindled since the crackdown.
Monasteries have been raided and hundreds of monks are thought to have been detained. Pictures from Burma show ransacked monasteries with pools of blood on the ground.
The BBC's Jonathan Head says Burma's rulers have turned their backs on the world and the torrent of outrage their actions have provoked.
The first opportunity to communicate that outrage will be when the UN special envoy, Ibrahim Gambari, arrives in Burma in the next day or two, when he will try to persuade the generals to put a stop to the crackdown.