* Thousands gather outside state-owned telecom group
* Police tighten security near Government House
* Protesters vow to occupy more ministries
Police tightened security in Thailand's capital on Saturday as about 2,000 protesters rallied outside a state telecommunications group and vowed to occupy Prime Minister Yingluck Shinawatra's office to paralyse her administration.
Faced with dwindling support, demonstrators have started to up the ante and briefly occupied the headquarters of the army on Friday, urging it to join their side in a complex power struggle centred on the enduring political influence of Yingluck's billionaire brother, ousted prime minister Thaksin Shinawatra.
"On Sunday, brothers and sisters, we will announce our victory and our defeat of the Thaksin regime," protest leader Suthep Thaugsuban told a rally of thousands late on Friday.
A crowd of about 2,000 massed outside state-owned TOT Plc, a telecoms company. Others huddled under umbrellas at a protest base in Bangkok's historic district where many said they planned to stay until marching to various ministries on Sunday.
Suthep has called on supporters to surround the headquarters of the national and city police, along with the heavily barricaded Government House and even a zoo on Sunday. "We need to break the law a little bit to achieve our goals," said Suthep, a deputy prime minister under the previous government that Yingluck routed in a 2011 election.
The threats heighten a nearly decade-long conflict that broadly pits Thailand's traditional establishment of top generals, royalists and the urban middle class against the mostly rural, northern supporters of Thaksin.
Thaksin remains intensely polarising.
The populist leader was removed in a 2006 military coup and convicted two years later of graft -- charges he says were politically motivated. He remains closely entwined with government from self-imposed exile, sometimes meeting with Yingluck's cabinet by webcam.
Suthep has urged his followers to shut a government administrative complex on Saturday and by Sunday, move on the ministries of labour, foreign affairs, education and interior. A few thousand protesters gathered peacefully at the government complex on Saturday, many sitting in groups munching food.
But it remains unclear whether he can besiege multiple government offices. Protester numbers peaked at more than 100,000 last Sunday. By Friday, Suthep drew just 7,000 people to his regular evening speech, police said.
"We will not allow protesters to seize Government House, parliament or the national police headquarters. I don't believe protesters can get close to Government House because we have roadblocks and other blockades in place to stop them," National Security Chief Paradorn Pattanathabutr told Reuters.
Hundreds of police spilled out of buses and vans near Government House. One police official said about 5,000 police would reinforce the area over the day.
Suthep, a silver-haired politician from Thailand's south, just weeks ago resigned from the opposition Democrat Party, which has not won an election in more than two decades and is backed by Bangkok's royalist elite.
He has called for a "people's council," which would select "good people" to lead the country, effectively suspending Thailand's democratic system.
A defiant Yingluck has rejected that as unconstitutional and has repeatedly ruled out new elections.
The protesters have accused the government of acting above the law after senior members of the ruling Puea Thai Party refused to accept a Nov. 20 Constitutional Court ruling that rejected their proposal to make the Senate fully elected. Puea Thai has argued the judiciary has no right to intervene in the legislative branch.
The measure would have strengthened Yingluck's government by giving her strong support in vote-rich northern Thailand.
The ruling casts a spotlight on Thailand's politicised courts, which annulled an election won by Thaksin in 2006 and brought down two Thaksin-allied governments in 2008 after similar protests. Members of Yingluck's party have said the judiciary had no right to intervene in the legislative branch.
Yingluck has sought to keep her distance from the issue, never openly rejecting the court ruling and stressing that she is not head of the Puea Thai Party.
That has done little to allay the flag-waving, whistle-blowing protesters.
Democrat leader Abhisit Vejjajiva, a former prime minister, said Yingluck had "acted above the law" by rejecting the Constitutional Court. He delivered a letter to the U.S. embassy on Friday explaining why she must go.
"May our whistles be heard to Washington D.C.," Korn Chatikavanij, a senior Democrat and former finance minister, told the crowd.
The protests are the biggest since red-shirted Thaksin supporters paralysed Bangkok in April-May 2010 in a period of unrest that ended with a military crackdown in which 91 people, mostly Thaksin supporters, were killed.
The Oxford-educated Abhisit and his then-deputy Suthep both face charges of murder in connection with that unrest, accused of allowing soldiers to open fire on protesters.
Bangkok is on edge again. Since Monday, demonstrators have surrounded ministries and rallied in a commercial district and outside Yingluck's party headquarters, though the number of protesters appears to have declined steadily through the week.
Friday's brief invasion of the Royal Thai Army's headquarters, which lasted about three hours and ended peacefully, illustrates how the protesters see the military as a potential ally because of its attempts to intervene against governments led or backed by Thaksin over the last decade.
Army chief Prayuth Chan-ocha, however, told protesters not to drag the military into politics. "We hope all sides will unite and not use the army as a tool," he said.