At least one person was shot dead and 10 were wounded after anti-government protesters clashed with supporters of Thai Prime Minister Yingluck Shinawatra on Saturday, the first bloodshed in a week of demonstrations aimed at toppling her administration.

Fighting intensified after anti-government protesters attacked a bus they believed was full of government "red shirt" supporters. They also smashed the windshield of a taxi carrying people wearing red shirts, a pro-government symbol, and beat two people, one unconscious, police and Reuters witnesses said.

As darkness fell, gunfire erupted outside a sports stadium in Bangkok's Ramkamhaeng area where about 70,000 red-shirted supporters of Yingluck and her brother, ousted prime minister Thaksin Shinawatra, had gathered for a rally.

A gunman fired into Ramkamhaeng University, where hundreds of anti-government protesters had retreated after trying to block people from entering the stadium, witnesses said.

It was not immediately known who fired the shots, but the violence raises the stakes in a conflict that broadly pits Bangkok's middle class, royalists and business leaders against the mostly rural, northern supporters of Yingluck and her billionaire brother Thaksin.

"Students were inside the compound of the university when all of a sudden a gunman fired several shots through the fence of the university injuring many students," Attiwat Nakpao, deputy leader of a Ramkamhaeng University student group, told local media.

At least five people suffered gunshot wounds, five others were injured by knives or rocks, and one had been shot dead, officials at the nearby Ramkamhaeng and the Dr Panya General Hospital told Reuters.

The US embassy in Bangkok expressed concern about the rising political tension.

With a Sunday deadline set by demonstrators for the ousting of the government, police called for military backup to protect parliament and Yingluck's office, Government House, where protesters tore down stone and razor wire barriers ahead of a planned move to occupy it on Sunday.

Demonstrators have started to up the ante and briefly occupied the headquarters of the army on Friday, urging it to join them in a complex power struggle centered on the enduring political influence of Yingluck's billionaire brother, Thaksin.

Those attacked by the crowd were accused of being "red shirts", ardently loyal supporters of Yingluck and Thaksin, who gathered in their thousands at the Rajamangala stadium to ward off any coup attempt against the government.

"Early on in the evening anti-government protesters pulled at least two red-shirted men from their motorbikes. One guy was stripped of his shirt. The crowd then burned his shirt while others kicked him in the chest," said one witness.

"Later on, at the back of the university, students clashed with red shirt supporters in the neighborhood," he added. "Some red shirt supporters threw stones at the students from their houses. Both sides were armed with sticks and rocks."

Protest leader Suthep Thaugsuban told supporters late on Friday to surround the headquarters of the national and city police, along with 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 in the previous government, routed by Yingluck in a 2011 election.

Thaksin remains intensely polarizing. He was removed in a 2006 military coup and convicted two years later of graft, on charges he calls politically motivated. He is closely entwined with the government from self-imposed exile, sometimes meeting with Yingluck's cabinet by webcam.

A crowd of about 2,000 people massed outside state-owned telecoms companies on Saturday and Suthep has urged his followers to move on the ministries of labor, foreign affairs, education and interior. It remains unclear whether he has the numbers to besiege multiple government offices.

National Security Chief Paradorn Pattanathabutr told Reuters Government House or the police headquarters would not be seized. Seventeen battalions of 150 soldiers each, along with 180 military police, all unarmed, will reinforce security on Sunday, said the Centre for the Administration of Peace and Order.

Suthep has called for a "people's council", which would select "good people" to lead the country, effectively suspending Thailand's democratic system. Yingluck has rejected that step as unconstitutional and has repeatedly ruled out a snap election.

The protesters have accused the government of acting unlawfully, after senior members of the ruling Puea Thai Party refused to accept a November 20 Constitutional Court ruling that rejected their proposal for a fully elected Senate, which would have boosted the party's electoral clout. Puea Thai says the judiciary has no right to intervene in the legislative branch.

The ruling casts a spotlight on Thailand's politicized courts, which annulled an election won by Thaksin in 2006 on a technicality and later dissolved his Thai Rak Thai Party for electoral fraud. Its next incarnation, the People's Power Party, suffered the same fate. Nearly 150 executives of both parties were banned for five years.

Democrat leader Abhisit Vejjajiva, a former prime minister, said Yingluck had "acted above the law" by rejecting the Constitutional Court ruling.

Chaturon Chaisang, education minister and a close ally of Thaksin, said those accusations lacked rationale.

"The government, the prime minister and the cabinet have said nothing about accepting or not accepting the Constitutional Court decision," Chaturon told Reuters, adding that any attempts to use the courts to try to topple the government were unlikely to succeed this time.

The protests are the biggest since red-shirted Thaksin supporters paralyzed 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.-Reuters