If it had happened elsewhere, it might have been dismissed as a teenage prank.

A couple of 15-year-olds last year hung a Tibetan banner on the wall of their classroom next to portraits of Mao Tse-tung and Deng Xiaoping. They drew Xs over the faces of the former Chinese leaders and scrawled “Long Live the Dalai Lama” on the wall.

But in China, the incident was taken dead seriously. Three boys who attend a Tibetan school in Sichuan province were arrested, and one of them, who confessed to being the ringleader, was held and interrogated for a month.

This year, the 50th anniversary of the Dalai Lama’s exile to India, the boy is under virtual house arrest -- by his own parents.

“They want to make sure he doesn’t do anything like that again. They don’t want him to get arrested again or hurt,” said a relative, who asked not to be quoted by name.

As the traditional season for protests against Chinese rule begins, many of China’s more than five million Tibetans are in a similar quandary. As sure as the melting of the snow on the Tibetan plateau, protests erupt about this time of year as Tibetans mark the anniversary of a failed uprising that began March 10, 1959, and led to their spiritual leader’s exile. But with the special anniversary this year, the Chinese have taken extraordinary security measures, hoping to prevent a recurrence of last year’s protests, the most violent in decades.

Exact numbers are difficult to come by in China, but residents of the Tibetan areas say that tens of thousands of paramilitary troops have been deployed. Telephones are tapped; cellphones and Internet connections disrupted.

Foreigners are barred from entering not only the Tibetan autonomous region, but heavily Tibetan areas in Qinghai, Sichuan and Gansu provinces.

The mountain roads leading into Tibetan villages are clogged with armored personnel carriers and buses filled with riot police. The security forces make their presence felt by cruising up and down main streets, or conducting training in as public a way as possible.

“They made a picture of a man and are using itfor target practice. They do it to scare us,” said Tashi, a 20-year-old student from Ganzi (known to Tibetans as Kardze), in Sichuan province. Like many Tibetans, Tashi refuses to be identified by anything other than a first name for fear of retaliation for speaking out.

In Lhasa, the Tibetan capital, security cameras are ubiquitous.

“You feel like you’re being watched all the time. It feels very uncomfortable,” said Tsering, a 38-year-old Tibetan construction worker from Lhasa, who left late last year.

Tibetans say they would like to do something to commemorate the victims of last year’s violence, who they say number more than 200. (The Chinese counter that 21 died and that most were Chinese.)

“We would like to do a candlelight demonstration for the people who died, but we are afraid. So we will keep silent,” said Tashi.

But to demonstrate or not has become a subject of great debate among young Tibetans.

“Our teachers have told us not to be stupid and talk about freedom,” said a 19-year-old student from Tongren, a monastery town also known as Repkong, in Qinghai province, that has been a hotbed of ethnic unrest.

“Everybody is afraid. There are armed police all over our town,” he said.

The problem for Tibetans is that the Chinese treat any expression of Tibetan identity -- even waving a flag or posting a portrait of the Dalai Lama -- as criminal activity.

Human Rights Watch on Monday issued a detailed report saying that China had arrested thousands of Tibetans on the vaguest of charges, failing to disclose what crimes had been committed or where people were being held.

And today, the Dalai Lama was expected to launch his own harsh criticism of China. According to an advance copy of his 50th anniversary speech released to wire services by his aides, the spiritual leader says that China has launched a “brutal crackdown” in Tibet in the last year and that Tibetan culture and identity are “nearing extinction.”

So far only a few Tibetans have dared defy the stifling security that has blanketed the Tibetan areas from Lhasa to Qinghai, Sichuan and Gansu provinces.

Early Monday, a police car and a fire engine were bombed with homemade explosives after a dispute at a lumberyard in the Tibetan enclave of Golog, in Qinghai province. No injuries were reported.

Last month, a 24-year-old Tibetan monk in Sichuan set himself on fire to protest Chinese rule.

But most protests have been silent, passive affairs -- for example, many Tibetans refused to celebrate during their own New Year’s holiday in commemoration of those killed or imprisoned in last year’s uprising.

Even the ethnic Chinese are cowed by the heavy hand of security, which has all but killed off the tourist trade in places like Lhasa, the Himalayan city so beloved by backpackers and seekers of spiritual solace.

“It is very intense. Our telephones and cellphones are under surveillance,” said the Chinese manager of a small inn, which, like most others in the city, had no customers.

“Paramilitary are everywhere, at all the major intersections and the little streets too.”

The Chinese government has defended the measures as justified in light of last year’s riots, in which hundreds of Chinese-owned businesses in Lhasa were torched.

“Is it not crucial for the central government to take action to maintain social stability in Tibet, to protect the innocent from harm?” said an English-language commentary by the state-run New China News Agency. “Any other government in the world would be on alert too, had they been in China’s shoes.”

Speaking Monday in Beijing on the sidelines of the National People’s Congress, the annual legislative gathering, Fu Hongyu, a top official of the Ministry of Public Security, acknowledged the heightened security.

“We have made due deployment and tightened controls at border ports and key areas and passages along the border in Tibet,” said Fu.

Champa Phuntsok, the Chinese-installed chairman of the Tibetan Autonomous Region, appears confident that this year, Tibetans will behave.

“Riots like those last March won’t happen again,” he predicted at the People’s Congress.

barbara.demick@latimes.com