The rally by the People’s Alliance for Democracy — also known as the Yellow Shirts — and an associated fringe group are raising tensions in a country still recovering from political violence last year that turned parts of the capital into a war zone. Police on Monday arrested five men accused of plotting to bomb the protest.

The demonstrators set up a stage along a major street near the U.N.’s Asian headquarters and Government House, the prime minister’s office that the Yellows occupied for three months in 2008. By late afternoon more than 2,000 people were camped in the area and tents and toilets were being set up.

The protesters want the government to revoke a pact with Cambodia on settling border disputes; withdraw from the U.N. Education Scientific and Culture Organization World Heritage Committee, which approved Cambodia’s application for landmark status for a temple on the border; and force Cambodian residents off land the group claims should belong to Thailand.

The demands have already been rejected by Prime Minister Abhisit Vejjajiva, and the protest appears mainly aimed at putting pressure on the government ahead of elections that must be called by the end of the year.

Asked how long the rally would last, Chamlong Srimuang, an alliance leader, said: "I don’t know. It depends on the prime minister, not us. If he answers our demands, then we quit."

Thailand has seen a number of prolonged and often violent street protests since domestic politics became unhinged in 2006. That was the year a military coup deposed then-Prime Minister Thaksin Shinawatra following demonstrations by the Yellow Shirts calling for him to step down.

The alliance — which previously campaigned for the country’s electoral laws to be rewritten so rural voters would have less power — took to the streets again in 2008, staging its occupations of the prime minister’s office and Bangkok’s airports to call for the removal of two successive pro-Thaksin prime ministers.

The demonstrations and the removal of the Thaksin-allied governments by court decisions inspired a backlash movement by the United Front for Democracy Against Dictatorship — known as the Red Shirts — that culminated in last year’s protest that shut down the heart of Bangkok’s shopping district for more than a month and led to deadly clashes with the military.

On Monday, police arrested five men who were allegedly planning to set off bombs to disrupt the latest demonstrations. One was seized as he was allegedly preparing to place bombs at the site, and the others at a house where police found rocket-propelled grenades and launchers.

The Cambodian issue has its origins in a dispute between Cambodia and Thailand over land near a landmark temple on their border, but has evolved into a Thai domestic political issue.

The International Court of Justice ruled in 1962 that the 11th century Preah Vihear temple belongs to Cambodia, but the decision rankled Thailand, which still claims land around the temple.

The issue was virtually dormant until Cambodia applied in 2008 to UNESCO to have the temple declared a World Heritage site, an application backed by the government in power in Bangkok at the time.

Right-wing groups protested that the action threatened Thailand’s sovereignty, though their protests were seen as mainly a way of rallying opponents of the government. Both countries’ leaders, defending their patriotic credentials, then built up military forces at the border, which have engaged in several brief clashes in the past two years.