BANGKOK (AP) -- Protesters in Thailand stormed the grounds of the national army headquarters on Friday, asking the military to support their increasingly tense campaign to topple Prime Minister Yingluck Shinawatra.

In a letter addressed to the army chief, the protesters stopped short of calling for a coup but urged military leaders to "take a stand" in Thailand's spiraling political crisis and state which side they are on.

The crowd of 1,200 people stayed on the sprawling lawn of the Royal Thai Army compound for two hours before filing out peacefully. It was a bold act heavy with symbolism in a country that has experienced 18 successful or attempted military coups since the 1930s.

The most recent was in 2006, when the military ousted Yingluck's brother, former leader Thaksin Shinawatra, who is living overseas to avoid a corruption conviction but is central to Thailand's political conflict.

For the past week, thousands of anti-government protesters have marched in Bangkok in a bid to unseat Yingluck, whom they accuse of serving as a proxy for her billionaire brother. Thaksin is adored by much of the country's rural poor and despised by the educated elite and middle-class who accuse him of widespread corruption and other offenses.

Leaders of the protests say their goal is not just to force Yingluck out of office but to rid the country of Thaksin's influence in politics.

Asked if she planned to call early elections, Yingluck told the BBC on Friday that she didn't think snap polls would solve the country's problem.

"You have to ask (if) the protesters (would be) satisfied or not," Yingluck said.

The demonstrations have raised fears of new political turmoil and instability in Thailand and pose the biggest threat to Yingluck's administration since she came to power in 2011.

Protesters branched out to several spots on Friday, with another crowd staging a rally outside the headquarters of Yingluck's ruling Pheu Thai party, where hundreds of riot police stood guard to prevent them from entering.

A separate crowd of more than 1,000 people marched through central Bangkok to the U.S. Embassy. Opposition lawmaker Korn Chatikavanij, a former finance minister, delivered a letter to an official there denouncing Yingluck's leadership as illegitimate, in response to a statement from Washington that expressed concern about the protests.

Despite heavy security in several protest areas, the army headquarters was apparently not expecting the protesters, who had little trouble getting past the locked iron gate.

"From what I understand, there was a padlock on the gate. They broke it and let themselves in," army spokesman Sansern Kaewkamnerd said.

The compound is next to the United Nations' Asia-Pacific headquarters in Bangkok.

Yingluck has been reluctant to use force against the opposition-led protesters for fear of escalating the crisis and sparking bloodshed.

Security forces have done little to stop protesters who have spent the week seizing government buildings and camping out at several of them in an effort to force a government shutdown while asking civil servants to join their rally.

Crowd sizes peaked Sunday at over 100,000 and have dwindled in recent days to tens of thousands, but organizers have kept each day dramatic by targeting new and different seats of power.

Crowds of protesters have occupied the Finance Ministry since Monday and others have remained holed up since Wednesday at a sprawling government complex that houses the Department of Special Investigations, the country's equivalent of the FBI. On Thursday, the demonstrators cut power at Bangkok's police headquarters and asked police to join their side.

Yingluck has publicly pleaded for the protesters to stop and asked leaders of the movement to negotiate.

"Please call off the protests for the country's peace," Yingluck said Thursday. "I'm begging you."

But protest leader Suthep Thaugsuban, who resigned as an opposition Democrat Party lawmaker to lead the protests, says he will not negotiate. 

Suthep and other protest leaders have called for bigger crowds to join the campaign over the weekend.

"Come join us, as many as you can. Within two days, we will achieve victory," Satit Wongnongtoey, a protest leader, told crowds gathered at nightfall at Democracy Monument in Bangkok. "We've come this far. We will not go back."

Thaksin, who lives in Dubai to avoid serving a jail term for a corruption conviction he says was politically motivated, is a highly polarizing figure in Thailand. Before Thaksin was toppled in a coup -- for alleged corruption, abuse of power and insulting the nation's revered king -- he won over Thailand's rural underclass by introducing populist policies designed to benefit the poor. His political movement grew to become the most successful in modern Thai history.

But his opponents saw him as arrogant and a threat to democracy and their own privileges. The country has been gripped by sometimes violent protests by both sides since 2006.