BANGKOK (AP) -- Tens of thousands of anti-government protesters marched through the streets of Thailand's capital Saturday, reviving their whistle-blowing, traffic-blocking campaign to try to force the resignation of the country's prime minister.
The protest came after a lull in anti-government rallies and amid growing concern of violence between opponents and supporters of embattled Prime Minister Yingluck Shinawatra. It also came a day before a key vote to elect a new Senate.
Yingluck's opponents have tried a variety of tactics for the past four months to force her ouster. They have blocked Bangkok's major intersections, stormed government offices and most recently transformed the city's sprawling Lumpini Park into a messy protest headquarters overrun with tents and sleeping bags.
Saturday's crowds marched from Lumpini Park, in the central business district, to the city's historic quarter to press demands that the government yield power to an interim appointed council to oversee reforms before new elections. Protesters say Yingluck is a proxy for her brother, former Prime Minister Thaksin Shinawatra, who was ousted in a 2006 coup after being accused of corruption and abuse of power.
"We march today to call for an end to the Thaksin regime, and show that the power truly belongs to the people," said a protest leader, Thaworn Senniem.
A group of several hundred protesters forced their way into the prime minister's office compound, Government House, in a symbolic show of defiance. The compound has been largely deserted by officials since the protests started.
The march was the first major rally since Thailand's Constitutional Court ruled March 21 to nullify last month's general election, a ruling cheered by protesters and criticized by Yingluck's supporters as the latest sign of judicial bias against her.
"The fact that the election has been nullified means that our campaign is successful," Thaworn said. "Now we must finish the job with reforms."
Yingluck has refused to resign and had called the Feb. 2 early elections to receive a fresh mandate. Her ruling Pheu Thai party and its predecessors have easily won every national election since 2001. It had been expected to win again in February, especially because the opposition Democrat Party boycotted the election.
Election officials say it will take at least three months for a new vote to be held, prolonging Thailand's political paralysis.
Yingluck's supporters, known as the Red Shirts, have generally kept a low profile during the anti-government protests. However, as Yingluck's government comes under greater threat of legal action that might force it from office, they have said they are prepared to respond with force.
On Monday, Yingluck is due to submit her defense to the National Anti-Corruption Commission for a case her supporters call politically motivated that could lead to her impeachment.
If the commission decides to indict Yingluck and forward the case to the Senate for an impeachment vote, government supporters have vowed to rise up in her defense. The case accuses Yingluck of dereliction of duty over the government's flagship rice subsidy program, which has run up huge losses.
The current Senate is pro-Thaksin, but that could change in Sunday's election to fill 77 seats in the 150-seat Senate. The remaining seats are appointed, and a government attempt to make the Senate a fully elected body was one of the triggers for the unrest that started in November.
Yingluck's Red Shirt supporters have vowed to stage their own mass rally next Saturday, though they have not yet said whether it will be held in the capital, which many fear could lead to clashes between the two sides. The sporadic violence over the past four months has left at least 23 people dead and hundreds hurt.
Thailand has seen political conflict since 2006, when Thaksin was ousted by a military coup. Thaksin's supporters and opponents have since taken to the streets for extended periods in a power struggle.
Associated Press writer Jocelyn Gecker contributed to this report.
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