Burma's military leaders have imposed a dusk-to-dawn curfew in the main cities Rangoon and Mandalay, which have seen escalating anti-government protests.
Armed troops have now been deployed after tens of thousands of Buddhist monks and civilians again defied the army's warning to stay off the streets.
World leaders at the UN General Assembly have condemned the situation.
President George W Bush said the US was "outraged" by Burma's human rights record and announced further sanctions.
These include US visa restrictions on the Burmese leadership and their financial backers, Mr Bush said.
Analysts say further sanctions are unlikely to be effective without the support of Burma's main trade partners, China and India.
Vow to continue
The night curfew, along with a ban on gatherings of more than five people, were announced through loudspeakers on military lorries driving through Rangoon and Mandalay.
The restrictions will be in effect for 60 days, according to the announcement.
1. Shwedagon Pagoda. Tens of thousands of protesters, led by monks, gathered for the march
2.Sule Pagoda. Students joined the protest, passing by city hall
BBC South Asia correspondent Jonathan Head says the feared military response to what has become a mass uprising against Burma's rulers has drawn closer, with the deployment of armed troops at some of the focal points for the recent protests.
State television has repeated warnings to Burma's people not to join the demonstrations, and to the Buddhist monks not to meddle in politics.
But for an eighth day the monks once again led tens of thousands of people through Rangoon and other towns, calling for democracy and an end to military repression.
Joining them were workers, actors and politicians from the opposition National League for Democracy party led by Aung San Suu Kyi, who remains under house arrest.
Some chanted "we want dialogue" while others simply shouted "democracy, democracy".
People were also carrying flags, including some bearing the image of a fighting peacock used by students during the 1988 pro-democracy uprising, witnesses say.
Students were also openly marching, whereas during earlier demonstrations they had simply formed a chain and clapped.
The young monks leading this movement have vowed to keep up their protests.
But our correspondent says that now looks certain to put them on a collision course with the army.
The junta, which repressed the 1988 protests by killing some 3,000 people, finally broke its silence over the mounting protests late on Monday, saying it was ready to "take action".
15 Aug: Junta doubles fuel prices, sparking protests
5 Sept: Troops injure several monks at a protest in Pakokku
17 Sept: The junta's failure to apologise for the injuries draws fresh protests by monks
18-21 Sept: Daily marches by monks in Burmese cities gradually gather in size
22 Sept: 1,000 monks march to the home of Aung San Suu Kyi in Rangoon
23 Sept: Up to 20,000 march in Rangoon
24 Sept: New Rangoon march draws at least 50,000 and 24 other towns join in
Speaking at the UN General Assembly in New York, Secretary General Ban Ki-moon urged Burma's military to show restraint in the face of the rising tide of protests.
President Bush announced a tightening of existing US economic sanctions against the military regime there.
The US has already imposed an arms ban on Burma, a ban on all exports, a ban on new investment and a ban on financial services.
Earlier, close ally China called for stability and the EU also urged the junta to show restraint and launch a process of real political reform.
The protests were triggered by the government's decision to double the price of fuel last month, hitting people hard in the impoverished nation.
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