(CBS/AP) Security forces fired warning shots and tear gas into massive crowds of demonstrators in Myanmar's biggest city Wednesday, while hauling away defiant Buddhist monks into waiting trucks - the first mass arrests since protests in this military dictatorship erupted last month.

About 300 monks and activists were arrested across Yangon after braving government orders to stay home, according to an exile dissident group, and reporters saw a number of monks, who are highly revered in Myanmar, being dragged into trucks.

A Norway-based dissident radio station, the Democratic Voice of Burma, said that one monk was killed and several injured in clashes in downtown Yangon. Britain's Sky News reported a hospital source in Myanmar had confirmed one death.

The junta had banned all public gatherings of more than five people and imposed a nighttime curfew following eight days of anti-government marches led by monks in Yangon and other areas of the country, including the biggest protests in nearly two decades.

A march toward the center of Yangon followed a tense confrontation at the city's famed Shwedagon Pagoda between the protesters and riot police who fired warning shots into the air, beat some monks and dragged others away. They also fired tear gas.

About 5,000 monks and 5,000 students along with members of detained opposition leader Aung San Suu Kyi's pro-democracy party set off from Shwedagon to the Sule Pagoda in the heart of Yangon, but were blocked by military trucks along the route.

Other protesters at the Sule Pagoda were confronted by warning shots.

Some carried flags emblazoned with the fighting peacock, a key symbol of the democracy movement in Myanmar. The march proceeded quietly with protesters praying rather than chanting.

About 100 monks stayed behind at the eastern gate of the Shwedagon, refusing to obey orders to disperse after riot police there failed to dislodge them with tear gas, batons and warning shots.

Witnesses said an angry mob at the pagoda burned two police motorcycles.

A branch of Suu Kyi's National League for Democracy exiled in Thailand said 300 people had been arrested in Yangon, most of them in a western suburb of the city. The number could not be independently confirmed.

If the military responds to new protests with force, it could further isolate Myanmar from the international community. It would almost certainly put pressure on Myanmar's top economic and diplomatic supporter, China, which is eager to burnish its international image before next year's Olympics in Beijing.

President Bush announced new U.S. sanctions against Myanmar during a speech at the U.N. on Tuesday, accusing the military dictatorship of imposing "a 19-year reign of fear" that denies basic freedoms of speech, assembly and worship. (Video)

Mr. Bush said the U.S. would tighten economic sanctions on leaders of the regime and their financial backers, and impose an expanded visa ban on those responsible for human rights violations and their families.

British Prime Minister Gordon Brown called for an urgent meeting of the U.N. Security Council to discuss what further measures could be taken against the military junta.

Brown urged the U.N. to send an envoy to the country immediately, to monitor the situation and ensure the junta is aware that "any trampling of human rights" will not be accepted.

David Miliband, the U.K. Foreign Secretary, said Wednesday morning after a conversation with the British Ambassador to Myanmar that the "situation remains very tense".

"It's important that they (junta) continue to understand that restraint must remain the order of the day," Miliband told reporters, adding that the military regime seemed at least cognizant of the fact that the world was watching very carefully.

"The international community is a factor in the discussions going on there," he said.

"In contrast to 20 years ago, there is a much freer flow of information. It's harder to hide. That is a very important part of the new diplomacy. Governments around the world need to know that their actions will be seen," said Miliband, who went on to describe democracy as "a human yearning that isn't going to go away".

(CBS/AP) But one exiled opposition leader said all the sanctions in the world would have little affect on the Myanmar's military rulers.

Interviewed by Sky News, Dr. Win Naing, the chairman of the U.K. branch of the National League for Democracy, said simply, "this is not enough".

"We need urgent, concrete support from the whole international community… We need intervention," he said, urging foreign nations to send troops into Myanmar to defend the protesters.

Soldiers with assault rifles earlier blocked all four major entrances to the soaring pagoda in downtown Yangon, one of the most sacred in Myanmar, and sealed other flash points of anti-government protests.

A comedian famed for his anti-government gibes became the first well-known activist rounded up following the protests.

Zarganar, who uses only one name, was taken from his home overnight by authorities. His family said Wednesday they were told he had been "called in for temporary questioning."

Zarganar, along with actor Kyaw Thu and poet Aung Way, led a group providing food and other necessities to the protesting monks. He had earlier been imprisoned twice and his comedy routines banned for their satirical jokes about the regime.

The fates of the actor and poet were not immediately known.

Myanmar's leaders warned monks to stop the protests after some 100,000 people joined marches in Yangon on Monday in the largest anti-government demonstrations since a 1988 pro-democracy uprising was violently suppressed.

The junta imposed the 9 p.m.-to-5 a.m. curfew and ban on public assembly after 35,000 people monks and their supporters defied the warnings to stage another day of protests Tuesday.

The demonstrations started Aug. 19 after the government hiked fuel prices in one of Asia's poorest countries. But they are based in deep-rooted dissatisfaction with the repressive military rule that has gripped the country since 1962.

In Mandalay, Myanmar's second largest city, more than 800 monks, nuns and laymen played a cat-and-mouse game with some 100 soldiers who tried to stop them marching from the Mahamuni Paya Pagoda, which they had tried to enter earlier.

"We are so afraid, the soldiers are ready to fire on civilians at any time," a man near the pagoda said, asking that his name not be used for fear of reprisals.

If monks who are leading the protests are mistreated, that could outrage the predominantly Buddhist country, where clerics are revered. But if the junta backs down, it risks appearing weak and emboldening protesters, which could escalate the tension.

There are about 500,000 monks and novices in Myanmar.

When faced with a similar crisis in 1988, the government brutally suppressed a student-led democracy uprising. Soldiers shot into crowds of peaceful demonstrators, killing thousands.

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