Buddhist monks protest the military junta in Rangoon, cheered on by supporters. About 700 staged a similar show of defiance in the second-largest city, Mandalay. (Associated Press)

BANGKOK, Sept. 25 -- Burma's military rulers imposed a nighttime curfew and banned assemblies Tuesday after thousands of Buddhist monks defied warnings and mounted another day of pro-democracy protests to the cheers of crowds in the streets of Rangoon.

Although Tuesday's demonstration was allowed to proceed peacefully, several truckloads of soldiers and armed police were seen taking up positions in Burma's largest city late in the day, according to news agency reports and videos e-mailed out of the isolated Southeast Asian country.

The ban on assemblies and the appearance of reinforcements, including anti-riot troops carrying shields and truncheons, suggested that the military junta may be preparing to crack down despite appeals from around the world that it avoid using force and enter into negotiations with its opponents.

Addressing the annual U.N. General Assembly, President Bush announced that he will impose new economic restrictions on Burmese leaders and their financial backers and expand a U.S. visa ban on those deemed responsible for "the most egregious violations of human rights" as well as their families.

After a day of protest by an estimated 10,000 monks and lay supporters, some shouting "Democracy, democracy," junta supporters were seen driving around Rangoon warning via loudspeakers that "action" would be taken against anybody who continued to support the demonstrations, news agency reports said. Others announced a 9 p.m.-to-5 a.m. curfew in Rangoon and Mandalay, Burma's two largest cities, and said gatherings of five or more people were banned, setting the stage for confrontation if the monks continue to protest, the reports said.

"A crackdown is imminent," predicted Bertil Lintner, a veteran Burma specialist based in neighboring Thailand.

Similar protests in 1988 were put down by soldiers firing weapons into crowds of demonstrators, killing several thousand. But this time, security forces have remained in the background during more than a week of sustained anti-government agitation that has built into the most serious challenge to the military junta since the 1988 disturbances.

The junta warned on government-controlled television Monday night that security forces could step in if the current wave of demonstrations did not come to a halt. The threat followed a day-long protest march in Rangoon estimated to have included more than 50,000 people, perhaps up to 100,000, which was much larger than previous demonstrations and several times larger than Tuesday's march.

At the same time, the religious affairs minister, Brig. Gen. Thura Myint Maung, ordered senior Buddhist leaders to rein in younger monks leading the charge in the streets. "If the monks go against the rules and regulations in the authority of Buddhist teachings, we will take action under existing laws," state television quoted him as saying.

In what could be a taste of things to come, several hundred monks protesting in the northwestern city of Sittwe were attacked with tear gas and roughed up by security forces, the Reuters news agency reported. Others were reportedly arrested, sparking anger among their fellow monks in Rangoon.

The protests started Aug. 19, set off by a stiff rise in fuel prices. But they have escalated since then into a head-on political challenge against the military leadership that has run Burma, also called Myanmar, for most of the past half-century. Spearheaded by the Buddhist monks who are revered by Burma's approximately 50 million inhabitants, the demonstrations in recent days also have broadened to embrace lay students and members of the National League for Democracy, the political party headed by Nobel Peace laureate Aung San Suu Kyi.

The military junta has kept Suu Kyi under virtual house arrest and prevented her party from taking power despite its victory in elections in 1990. Reuters reported Tuesday that Suu Kyi, who has become a symbol for many of the protesters of their longing for democracy, was taken to a prison Sunday in an attempt to prevent her from emerging as a leader of the new antigovernment campaign. In a brief appearance at the gate of her home Saturday, she drew cheers from hundreds of protesters who were allowed to approach her residence.

The junta and its leader, Gen. Than Shwe, have been urged to abandon their exclusive grip on power as public concern over the increasingly tense situation surges across Asia and beyond.

"Americans are outraged by the situation in Burma, where a military junta has imposed a 19-year reign of fear," Bush said at the United Nations. "Basic freedoms of speech, assembly and worship are severely restricted. Ethnic minorities are persecuted. Forced child labor, human trafficking and rape are common. The regime is holding more than 1,000 political prisoners, including Aung San Suu Kyi, whose party was elected overwhelmingly by the Burmese people in 1990."

Bush called on other nations "to use their diplomatic and economic leverage to help the Burmese people reclaim their freedom." Although he did not mention any country by name, it was a message aimed particularly at China, the key trading partner and ally of the Burmese government.

China has not joined the chorus of condemnation. Instead, it reiterated its refusal to pressure for change in public. A Foreign Ministry spokesman said the Chinese government hopes Burma's rulers can "maintain stability and resolve the issue in its own way," according to news agency reports from Beijing.

The United States imposed stringent economic sanctions on Burma in 1997 and amplified them in 2003. But some human rights activists who closely track the issue said the latest sanctions may be more effective if the administration follows its own model in cutting off illicit money held by North Korea in foreign banks. Bush and White House officials did not discuss in detail how the new restrictions would work but said they would target specific individuals as opposed to the general sanctions now on the books.

Tom Malinowski of Human Rights Watch has consulted with administration officials on the matter. "Even though the generals in Burma are profoundly isolated from their own people and the world, they still have to bank somewhere and that makes them vulnerable," he said. "There's a vulnerability that's never been exploited by the international community. If they can't bank anywhere, they can't buy things, including guns."

Malinowski added that leaders of the junta may be surprised to find their access to cash cut off.

"It will have an impact," he said, "when the wife of the leading general walks into his bedroom in the morning and starts screaming at him, 'What happened to our money?' "

Burma has occupied a prominent spot on the White House radar screen since first lady Laura Bush became personally upset about the situation. In recent weeks, she has called on U.N. Secretary General Ban Ki-moon to urge more action on Burma and summoned reporters to condemn the government -- unusually public moves by the first lady.

"What we're trying to do is . . . ratchet up the pressure on this regime, to get them to understand that there is a time now for a political transition and that they should be using the turmoil in the country as a vehicle for planning and achieving that transition, rather than trying to crack down on it and turn the clock back to a time that the Burmese people are no longer willing to tolerate," said national security adviser Stephen J. Hadley.

Staff writer Peter Baker at the United Nations contributed to this report.