''A 21-year-old man was shot dead by two bullets to his left side,'' said Boonchuay Pochantong, an official at a police station near Ramkhamhaeng University where unrest broke out earlier.
The violence followed a confrontation between a red-shirt security guard and a Ramkhamhaeng student.
It also came on the eve of a ''total seizure'' campaign announced by anti-government protest leader Suthep Thaugsuban. He is urging anti-government protesters to take control of all key government agencies today as part of a ''Victory Day'' push.
Nearly 3,000 troops will be deployed to reinforce security in Bangkok, Centre for the Administration of Peace and Order (Capo) spokesman Piya Utayo said.
''From tonight there will be soldiers out to take care of security,'' he said in a televised address, adding that 2,730 military personnel from the army, navy and airforce would take part.
Saturday's violence flared in front of the university just after 4pm when a group of red shirts on pick-up trucks bound for their rally site at Rajamangala Stadium appeared to taunt the anti-government protesters.
Both groups exchanged verbal abuse before a brawl broke out.
It was estimated that about 3,000 people were involved in the scuffle, with several people injured on both sides.
More violence erupted sporadically throughout the afternoon and early evening as protesters refused orders to retreat into the university compound.
A Bangkok Post reporter heard gunshots and what sounded like an explosion near the university at about 8pm.
A 29-year-old Cambodian worker and two university students were taken to hospital a short time later with bullet wounds.
About 9pm, a third male student was shot in front of Rajamangala national stadium.
The worker was apparently a bystander and was shot in the back by a stray round.
A witness said he saw a man firing a gun from within a group of red shirt guards who were gathering near the back entrance of the university.
The man fired into the air and also at the students.
A group of Ramkhamhaeng students also reportedly smashed a taxi window shortly after the initial clash when they spotted a passenger wearing a red shirt.
Some students were also seen using wooden clubs and other objects to vandalise a city bus being used by red-shirt supporters to get to the stadium rally site. About 600 police officers were rushed to the scene to restore order.
Ramkhamhaeng Road, from Lam Salee intersection to the front of the university, was closed to traffic last night.
Speaking at the Democracy Monument demonstration base, ex-Democrat MP and protest leader Sathit Wongnongtoey claimed that some police officers had taken off their uniforms and donned red shirts to attack Ramkhamhaeng students in front of the university.
The officers had set up barricades to create a siege situation, he claimed.
Mr Sathit called on demonstrators at Democracy Monument to move to the university to help the students.
He said if Capo fails to control the situation at Ramkhamhaeng, Prime Minister Yingluck Shinawatra and Deputy Prime Minister Pracha Promnok must take responsibility.
Ms Yingluck on Saturday affirmed authorities would not use force on demonstrators and called for talks to settle the political problems.
Police will handle the situation in accordance with the law, she said.
Red-shirt supporters nationwide yesterday began leaving their home provinces to join the United Front for Democracy against Dictatorship (UDD) rally at Rajamangala Stadium, vowing to ''protect democracy'' until the anti-government demonstrators end their campaign.
It followed a call by UDD leader Tida Tawornseth for red shirts to put on a show of force in the capital after Mr Suthep earlier told his supporters to lay siege to Government House and all key government offices today.
''I beg all red-shirt sisters and brothers to come out while the Pheu Thai Party is still the elected government and still has power. Or will you come when it is too late?'' Ms Tida said.
Jatuporn Prompan, a UDD leader, said the government should not let Mr Suthep besiege government agencies.
He said Mr Suthep was trying to entice the government into using violence so the army would be forced to intervene.
In Udon Thani on Saturday, Khon Rak Udon Club chairman Khwanchai Praipana, said he was leading about 1,000 people to join the UDD rally in Bangkok after ousted prime minister Thaksin Shinawatra talked to him over the phone and asked for support.
Thousands of anti-government protesters on Saturday occupied the compound of the state telecommunications firm TOT Plc, padlocked the doors of the Department of Special Investigation (DSI), and partially cut off electricity at CAT Telecom.
Speaking last night to the crowd at the government administrative headquarters in Chaeng Watthana Road which the protesters have occupied since Wednesday, Mr Suthep urged Bangkok residents to join the ''V-Day'' operation by going to government agencies near their homes at 10.45am on Sunday.
He said the protesters would seize 10 government installations, including Government House and the Royal Thai Police headquarters.
People in other provinces should converge at their respective provincial halls, he said.
Read the original story here.