KATHMANDU, Feb 1 (Reuters) - Nepal's Maoist rebels launched an attack on army barracks ahead of anti-king protests on Wednesday called by political parties to mark the first anniversary of the monarch's assumption of absolute power.

An army officer said casualties were feared from the raid in Palpa town, about 300 km (185 miles) west of Kathmandu, late on Tuesday. But he didn't have more details.

The fighting continued until dawn and the independent Kantipur radio station said prisoners from a nearby jail in the had been freed by the guerrillas.

King Gyanendra threw Nepal into turmoil on Feb. 1 last year when he fired the government, jailed politicians and suspended civil liberties, triggering widespread protests at home and drawing strong criticism from the international community.

The king justified his takeover saying it was needed to crush the Maoist insurgency that has left more than 13,000 people dead since 1996.

Nepal's seven mainstream political parties, citizens' groups and professional organisations have planned nationwide rallies to mark Wednesday as "black day".

Political parties also plan to hoist black flags and burn effigies and there are fears of violence if activists try to defy a ban on rallies in the heart of Kathmandu.

Hundreds of activists were detained across the country on the eve of the anniversary in an apparent bid to thwart demonstrations, politicians said.

"Police have picked up activists from their homes. But we will go ahead with our plans and converge in the heart of Kathmandu," said Krishna Prasad Sitaula, a spokesman of the Nepali Congress, the country's largest political party.

"HARD BATTLE"

At 9 a.m. (0315 GMT), hours before the planned protests, the king is due to address the nation but no indications were available on what he planned to say.

"He has ignored appeals from the international community and the people to restore democracy and is moving ahead alone on the path of his autocratic monarchy," said Bam Dev Gautam, a senior leader of the Communist Party of Nepal (UML).

"We will continue our protests. The people's movement will intensify. Though it will be a hard battle, the outcome will be rewarding," he said ahead of the protests.

Although some of the king's tough curbs have been eased, analysts say he is nowhere close to fulfilling his promises to crush the Maoists, bring peace and good governance, and then restore democracy within three years.

Anger is mounting and anti-king protests have escalated with tens of thousands of people taking to the streets this year.

The king has called municipal elections for Feb. 8 in what his ministers say is a first step towards restoring democracy.

But the political parties, who were not consulted about the plan and will boycott the vote, see the elections as another confrontational gesture designed to sideline them.

The United States said it remained troubled by developments in Nepal a year after the king seized power.

"Twelve months of palace rule have only made the security situation more precarious, emboldened the Maoist insurgents, and widened the division between the country's political parties and the king," the State Department said in a statement on Tuesday.

"We urge the king to return to democracy by initiating a dialogue with ... political parties. After one year of unsuccessful authoritarian rule, this is the best way to address the Maoist insurgency and to build a brighter future for Nepal's people," it said.