People react around a man after he was attacked by anti-government protesters near the stadium where pro-government red shirts are gathering in Bangkok November 30, 2013. An anti-government mob attacked civilians and vehicles near a stadium rally by supporters of Thai Prime Minister Yingluck Shinawatra on Saturday as tensions boiled over and protesters tore down barricades to prepare to occupy her offices. [Photo/Agencies]

A "red shirt" Thai government supporter was shot and killed early on Sunday, raising the death toll to two from political violence in Bangkok as protesters intensified a week-long bid to topple Prime Minister Yingluck Shinawatra.

Police called in military back-up to protect government buildings after fatal street clashes erupted between supporters and opponents of Yingluck and her billionaire brother, ousted former Prime Minister Thaksin Shinawatra, near a sports stadium where about 70,000 government supporters had gathered.

Thai police have started using tear gas on protesters outside the Government House in Bangkok.

The fighting is the latest in an intractable conflict that broadly pits Thailand's establishment of royalists, wealthy elites and the urban middle class against the poor supporters of Thaksin, who originate mostly from provinces north of Bangkok, the country's lowest-income regions.

By morning, streets near the stadium were littered with broken glass and rocks, a Reuters witness said. A red-shirt leader, Jatuporn Promphan, said four red shirts had been killed, but Reuters only confirmed one, 43-year-old red shirt guard Viroj Kemnak.

Forty-five people were wounded in the fighting, according to the government's Erawan emergency center.

Thousands of red shirts have begun to return by bus to their homes in northern Thailand, but their departure is unlikely to defuse Thailand's worst political crisis since April-May 2010, a period of unrest that ended with a military crackdown. In all, 91 people were killed, mostly Thaksin supporters.

BANGKOK BRACES FOR VIOLENCE

Yingluck, who won a 2011 election by a landslide to become Thailand's first female prime minister, has called on the protesters to clear the streets and enter into talks to avoid confrontation, saying Thailand's economy was at risk after demonstrators occupied the Finance Ministry on Monday.

Protest leader Suthep Thaugsuban has told demonstrators laws must be broken to achieve their goals and has urged them to surround the headquarters of the national and city police, ministries and the prime minister's office at Government House. Shopping malls shut as Bangkok braced for violence on Sunday.

Seventeen battalions of 150 soldiers each, along with 180 military police, all unarmed, were called in to boost security on Sunday ahead of a deadline the same day set by demonstrators for the ousting of the government.

Fighting intensified on Saturday after anti-government protesters attacked a bus they believed was full of government 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 witnesses said.

As darkness fell, gunfire erupted outside the sports stadium in Bangkok's Ramkamhaeng area, where the 70,000 backers of Yingluck and Thaksin had gathered for a rally in a show of support after a week of anti-government protests.

Around 8 pm, 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. One person was killed. It was not known who fired the shots.

Fighting raged in the area through the night.

At around 2 am, Kittisak Srisunthorn, 36, said he was shot in the arm while sitting with a group of red shirt guards.

"I heard homemade bombs, gunshots. People started to throw rocks and glass bottles. There were around one hundred people gathered. I didn't see any police," Kittisak told Reuters.

A crowd of about 2,000 people had massed outside state-owned telecoms companies on Saturday and Suthep has urged his followers to move on the ministries of labour, foreign affairs, education and the interior.

It is unclear whether he has the numbers to besiege multiple government offices.

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

National Security Chief Paradorn Pattanathabutr told Reuters that Government House and the police headquarters would not be seized.