Homemade bombs damaged police vehicles in a Tibetan part of western China on Monday, and authorities expanded a security cordon across the restive region ahead of the 50th anniversary of a failed revolt that sent the Dalai Lama into exile.

Armed police patrolled the streets outside Buddhist monasteries, which have been at the forefront of many protests. New checkpoints went up on previously open roads, the Internet and text messaging have been blocked, and the government ordered foreigners out of the mixed Chinese-Tibetan city of Kangding.

Tuesday is the anniversary of an uprising 50 years ago against Chinese rule that sent the Dalai Lama into exile and protests last year that became the most widespread, violent revolt by Tibetans in decades.

Chinese President Hu Jintao said that Tibet was basically stable and urged Tibetan politicians in Beijing to develop the long-lagging region economically to tamp down on separatism. "We should build a solid great wall to oppose the separatists, uphold the unity of the mother and advance Tibet from basic stability to lasting stability," Hu said in brief comments carried by state-run television.

Early Monday, small bombs ripped the emergency lights and roofs off a police car and fire engine at a remote timber farm in Qinghai province, the official Xinhua News Agency reported. No deaths were reported. A local official, surnamed Qi, confirmed the explosion but provided no other details.

But the blasts, made by "unsophisticated homemade explosives," came hours after a clash between locals and police who were inspecting vehicles at the Makahe timber farm, Xinhua said.

In recent weeks, China has increased the police and military presence in Tibetan areas -- a quarter of Chinese territory that stretches from Tibet to parts of Qinghai, Sichuan and Gansu provinces. Convoys of armored vehicles and sandbagged sentry posts have turned the region into something of an armed camp.

Armed police patrolled the streets outside the Rongwo monastery in Tongren in larger than usual numbers over the weekend, the Tibet affairs watching blog, High Peaks Pure Land, reported. The account was confirmed by a person at the Rebkong Arts Research Institute next to the monastery.

Rongwo is a historic center of Buddhist learning, and its monks protested last year. Buddhism lies at the heart of Tibetan identity, and monks are seen by many ordinary Tibetans as spiritual and community leaders. That veneration has also in part spurred monks to activism.

Both the 1959 uprising and last year's started in Lhasa, Tibet's capital. After Lhasa erupted in ethnic rioting last March 14, demonstrations spread across the region. Some of the worst violence occurred in strongly traditional communities in the mountains where Sichuan, Gansu and Qinghai converge and home to large Buddhist monasteries that have chafed under Chinese-imposed religious controls.

Access to Lhasa and the rest of Tibet, often tight for foreigners at the best of times, was effectively shut off late last month. Recent visitors have described armed police posted on rooftops and throughout the city with checkpoints on the outskirts.

Officials next door in Sichuan's Ganzi prefecture said they received an emergency notice from the provincial government, ordering foreigners -- including reporters -- out of Kangding, 1,000 miles (1,600 kilometers) east of Lhasa, the last corner of Ganzi to remain open and a gateway to the region.

"There is a special situation and we hope you can leave as soon as possible," Zhang Lijuan of the Ganzi prefecture foreign affairs office told Associated Press reporters in Kangding. "Normally, this is an open place and we would welcome you. But because of this special situation, it's not convenient."

Zhang would not say why the city was sealed but said Kangding was also temporarily sealed off after last year's demonstrations. She said she did not know when the area would reopen.

The streets of Kangding were busy as usual Monday. Police cordoned off traffic in front of the main square, where riot police and machine gun-toting soldiers now regularly march past.

The closing of Ganzi adds to the difficulties of verifying events in a huge region. In Beijing, a top police official said border controls in Tibet were tightened to prevent disruptions by supporters of the Dalai Lama, Xinhua reported.

"We have made due deployment and tightened controls at border ports, and key areas and passages along the border in Tibet," Fu Hongyu, political commissar of the Ministry of Public Security Border Control Department, was quoted as saying.

Despite the efforts, acts of protest continue. Dozens of monks from Gomang monastery in Aba marched last week, shouting "Long live the Dalai Lama" and "We want human rights," to show support for a monk who had set himself ablaze at a nearby monastery to protest religious repression, according to accounts from Phayul.com, a Tibetan Web site, and Students for a Free Tibet.