Beijing, Mar 13: The biggest protests by Tibetan monks in nearly two decades have rippled into Chinese provinces populated by Tibetans, as the government's tough response draws condemnation from international groups.

The demonstrations over past days have followed marches around the world to mark the 49th anniversary of an uprising against Communist rule in the remote, mountainous region that has become a focus for protest ahead of this year's Beijing Olympics.

While China has focused on condemning foreign-based critics of its presence in Tibet, the shows of bold defiance within its borders are likely to make security preparations for the Olympic Games an even bigger worry for officials.

"The reports of protests outside Lhasa show that Tibetans know the eyes of the world are upon them and are determined not to let the momentum drop," Matt Whitticase of the London-based Free Tibet Campaign told Reuters.

On Monday, 500 monks from Drepung monastery defied authorities by staging a rare march in the Tibetan capital of Lhasa, an act that the Chinese government called "an illegal activity that threatened social stability".

About 2,000 Chinese security personnel fired tear gas to try to disperse 600 monks from Sera monastery taking part in a second day of street protests in Lhasa, a source told Reuters.

They demanded the release of about a dozen fellow monks from Sera detained this month for waving a Tibetan flag and shouting pro-independence slogans, the source said.

"The demonstrations are the largest by monks since the 1989 protests that led to the imposition of martial law in Tibet's capital," the International Campaign for Tibet said in a statement.

Another rights group said about 400 monks from Lutsang monastery in the northwestern province of Qinghai, known in Tibetan as Amdo, protested on Monday and shouted slogans for their exiled spiritual leader, the Dalai Lama, to return.

The Dalai Lama fled to India after the failed uprising in 1959, nine years after People's Liberation Army troops marched into the predominantly Buddhist Himalayan region. The protesters shouted "Free Tibet!", the Free Tibet Campaign said on Thursday.

About 100 monks from Myera monastery in the neighbouring province of Gansu also protested on Monday, the rights group said, adding that police were investigating who was involved.

A source with knowledge of the protests quoted monks and witnesses as saying the sound of gunfire was heard outside the walls of monasteries. But no casualties have been reported.

Chinese Foreign Ministry spokesman Qin Gang said the protesters in Lhasa were "seeking to spark social turmoil". "This was carefully planned by the Dalai clique in a bid to separate Tibet and sabotage Tibetan people's normal life of stability and harmony," he told a regular news conference. The strife "could be a harbinger of further clashes between Tibetans and Chinese authorities in this Olympics year", said Mary Beth Markey, vice president of the International Campaign for Tibet.