Beijing  - China has placed Tibet under "de facto martial law" ahead of the 50th anniversary uprising against Chinese rule on March 10, 1959, activists said Friday.

The troop presence was to prevent protests like those that swept across Lhasa and other Tibetan regions last year, the London-based Free Tibet Campaign said.

Witnesses in Tibet reported strong army presence, military convoys, deployment of paramilitary police and roadblocks ahead of the anniversary, the group said.

However, protests increased "noticeably" despite the security measures, Free Tibet Campaign said.

"China's provocative troop deployments and surrounding of Tibetan monasteries has ensured that the stakes could not be higher in Tibet on the eve of next week's 50th anniversary of the Tibetan uprising and flight of the Dalai Lama into exile," Stephanie Bridgen, the group's director, said.

In view of China's determination to clamp down on protests, a "potential catastrophe" threatened, Bridgen added. China's paramilitary police already showed early that the were prepared to fire with "impunity" at Tibetan protestors, while Tibetans were determined to protest.

The activists urged foreign governments to "break their silence" to prevent a possible repeat of the bloody clampdown against demonstrators.

Last year's unrest was the worst in almost two decades. Rallies at the March 10 anniversary escalated in Lhasa on March 14 into anti-Chinese violence and spread to other Tibetan-settled areas. Since then, Tibet has been practically isolated, with foreign journalists only allowed to travel to the region in government-organized trips.

Over the past weeks, several foreign journalists were prevented from travelling to the region, the activists said. (dpa)