By Diego Ore and Brian Ellsworth

CARACAS (Reuters) - Venezuelan security forces raided the headquarters of an opposition party accused of fomenting nearly a week of violent protests, witnesses said, as the country expelled three U.S. diplomats on charges of conspiring with demonstrators.

Presumed military intelligence officers burst into the opposition Popular Will party office and attempted to forcibly remove several activists after throwing tear gas inside, according to party officials.

"The intelligence officers arrived and began to harass us," said party activist Adriangela Ruiz. "They threw tear gas, took computers and tried to take away several people."

The government has issued an arrest warrant for Popular Will's founder, Leopoldo Lopez, 42, the U.S.-educated opposition leader accused of murder and terrorism in relation to the violent demonstrations of the past week.

He has been the main instigator of the demonstrations that have energized Venezuela's opposition, but show few immediate signs they will achieve their goal of ending the government of socialist President Nicolas Maduro.

Student protesters have taken his lead and are now promising to continue demonstrating around the country.

Videos sent to the media by Popular Will, which could not be independently identified, showed men entering the party's premises, waving guns and kicking down a door.

Students protesting outside the building then prevented the gunmen from taking anyone away, a party worker said.

Two government officials contacted by Reuters said they had no information about the incident and also did not have any way of making authorized spokespeople available.

The Caracas protests have been limited to mostly upscale areas, with little evidence so far that Venezuelans will join the demonstrations en masse across the country of 29 million people. Even so, thousands were out in the streets again on Monday.

Lopez, whose whereabouts were unknown, promised via an online video to hand himself in on Tuesday and called on supporters to march with him to the Interior Ministry.

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"Let's all go dressed in white to one place. Then I will walk alone. I will not put any Venezuelan's life at risk," he tweeted on Monday.

PERSONAE NON GRATAE

After several days of blaming the violence on meddling by Washington, the Venezuelan government declared three U.S. diplomats personae non gratae, giving them 48 hours to leave the country on charges they were recruiting college students for the protests.

"They have been visiting universities with the pretext of granting visas," said Foreign Minister Elias Jaua, who often faced off against the police during his own days as a student demonstrator.

"But that is a cover for making contacts with (student) leaders to offer them training and financing to create youth groups that generate violence," he told reporters.

The U.S. State Department called the allegations "baseless and false," adding that Washington supported free expression and peaceful assembly in Venezuela and in countries around the world.

Venezuela has routinely expelled U.S. diplomats in recent years as the relationship between the two countries frayed during the 14-year rule of the late socialist firebrand, Hugo Chavez.

Critics dismiss such moves as theatrics used in times of national commotion to distract from more serious issues.

Venezuela's highly traded global bonds, which fluctuate sharply on civil unrest or political tension, remained near 18-month lows, though trading in U.S. markets was generally light due to the Presidents Day holiday.

Bank of America said in a research note to clients on Monday that the protests were unlikely to result in a change of government, recommending that investors take advantage of attractive yields on bonds that mature in coming years.

VIOLENCE ON BOTH SIDES

Complaints about acts of violence by both sides have piled up over six consecutive days of confrontations between police and demonstrators. Only 13 students were still reportedly detained after nearly 100 arrests in the past week.

Opposition activists say some of the detained students have been tortured, while videos and photos circulating online show uniformed men firing on protesters. Maduro insists police have been restrained in the face of provocation and attacks.

The reporters' trade union said 11 journalists have been arrested, some of whom were beaten and had their equipment stolen while covering the unrest.

Venezuelan photographer Gabriel Osorio said that on Saturday troops hit him in the head with a pistol, shot him with rubber bullets and broke one of his ribs.

"I was working. I wasn't throwing rocks," Osorio told a local newspaper. "I yelled: 'I'm with the press,' but that actually seemed to be what triggered their attack."

Government leaders have denounced violence by demonstrators linked to opposition marches, including vandalizing buildings and burning of trash along city avenues.

Hooded protesters have gathered outside the headquarters of state TV channel VTV for the past few nights, lighting fires in the streets and hurling stones and Molotov cocktails.

"If anyone thinks they're going to halt the activities of (state TV), they're sorely mistaken," said the channel's president, Yuri Pimentel.

(Additional reporting by Daniel Burns in New York, Javier Farias in San Cristobal, Carlos Garcia Rawlins and Jaczo Gomez in Caracas, Arshad Mohammed in Abu Dhabi; editing by Andrew Cawthorne and G Crosse)