BEIJING — Chinese security forces have stepped up a crackdown in Tibet's capital Lhasa, two years after protests marking a failed 1959 uprising erupted in deadly violence, police and reports said Thursday.

More than 400 people have reportedly been rounded up so far in the "strike hard storm" campaign launched earlier this month, which has worried residents on edge since the March 2008 unrest in the remote Himalayan region.

A policeman at a Lhasa precinct who asked not to be named told AFP on Thursday that the campaign was aimed at cracking down on Tibetan independence activities and ordinary crime.

"I don't know when we will end this campaign, but it could be at the end of March when this matter is over," said the policeman, referring to the sensitive anniversaries.

More than 1,500 extra police and security personnel had been deployed as of last week, with more than 4,100 rented apartments or homes searched, according to the Lhasa Evening News.

The newspaper said while more than 400 people had been taken into police custody, only 14 had been formally arrested on unspecified charges. It was not immediately clear if the others were released or remained in detention.

Lhasa residents said Thursday the city was tense due to the heavy police and military presence.

"There are armoured vehicles patrolling the streets... the television is always talking about the need to 'maintain stability'," said a retired woman who identified herself as Ceyang.

"We don't dare go out at night."

Police are carrying out identification checks of the city's migrant population as well as increasing routine traffic stops, the Lhasa Evening News reported.

"We must clear our eyes, clench our fists, grip our weapons and firmly prevent and severely strike at every separatist or destructive activity that harms national security and social stability," the Tibet Daily quoted Zhang Yixiong, the region's deputy Communist Party secretary, as saying this week.

"Officers and soldiers are working hard to uphold social stability, safeguard socialist law, the basic interests of the people and the unity of the motherland."

An uprising against Chinese rule of Tibet erupted on March 10, 1959 but was crushed by China within weeks, forcing the Dalai Lama, Tibet's spiritual leader, to flee into exile.

Protests took place on the anniversary of the uprising in 2008, escalating in subsequent days into violent riots across Tibet and neighbouring regions with significant populations of ethnic Tibetans.

China has said 21 people were killed by "rioters," while security forces killed only one "insurgent."

But the Tibetan government-in-exile says more than 200 people were killed and 1,000 hurt in the unrest and subsequent crackdown in the remote region.

In recent weeks, China has kept up an incessant stream of vitriol aimed at the Dalai Lama, accusing him of fomenting unrest and seeking Tibetan independence, charges that the 1989 Nobel Peace Prize winner has denied.

The Dalai Lama has "twisted the real situation in Tibet and distorted and attacked the central government's policy in Tibet to spread proposals on independence or semi-independence and separatism," foreign ministry spokesman Qin Gang told reporters Thursday.

The Buddhist monk was "damaging" China's national unity, he said.

On Wednesday, in a speech to mark the failed 1959 uprising, the Dalai Lama said he had done everything he could to explain that his goal was Tibetan autonomy within China -- and not full independence as Beijing alleges.

"Although I have clearly articulated Tibetan aspirations... we have not obtained any concrete result," said the 74-year-old Dalai Lama.

"Judging by the attitude of the present Chinese leadership, there is little hope that a result will be achieved soon."