Indonesia promised to safeguard westerners amid continuing anti-war protests as police said 10 Muslim radicals arrested for allegedly harassing foreigners could face a year in jail.

"We will be firm. It is the duty of the state to protect all personnel and assets of friendly countries," said top security minister Susilo Bambang Yudhoyono. He said that so far no major incidents have been reported.

Police on Monday arrested 10 members of the hardline Islamic Youth Movement in a Central Jakarta tourist district. They were said to have tried to force foreigners to sign a document condemning the US invasion of Iraq. Police said they face up to a year in prison if convicted of threatening or using force.

Yudhoyono played down warnings by the British and Australian embassies of an imminent terrorist attack in the country's second city of Surabaya. "There have never been indications that there would be a terrorist act there," he said. He speculated that the warnings could have been related to rumoured plans by some groups to conduct "sweeps" for Americans in the city.

The US and allied embassies have warned that the Iraq war could prompt further terrorist attacks in Indonesia following the Bali blasts last October which killed 202 people, mainly foreigners.

But Foreign Minister Hassan Wirayuda described the latest warning as "excessive." "It doesn't mean we underestimate [the threat]. We are working very hard but we ask countries to correct their travel warnings after the anticipated attack did not happen," Wirayuda said.

Wirayuda also pointed out that daily anti-war protests had been peaceful and orderly. "I think their suspicions are groundless." The government has strongly criticised the US-led attacks as a violation of international law but has promised to clamp down on any violent protests.

Demonstrations in the world's largest Muslim-populated nation have so far been relatively small and peaceful despite widespread anger at the US invasion. Protesters have symbolically sealed McDonald's restaurants and other US-franchised premises but no damage has been reported.

On Tuesday an estimated 1,000-1,500 people marched in the North Sumatra capital of Medan, according to police.

In Jakarta about 200 protesters, some shouting "America – King of Terrorists," rallied outside the US embassy. One group carried a mock coffin draped with the Stars and Stripes.

About 100 students from the state Islamic university in the restive eastern city of Ambon marched to the governor's office to protest at the US attack. They called for a boycott of US goods and services and urged the government to freeze diplomatic ties with Washington.