Protesters hurled petrol bombs at troops who fired rubber-coated steel bullets and tear gas back at them across the capital.
As night fell, a Bangkok hospital official said at least five people had been killed, four of whom were protesters and one was a Japanese cameraman for the Reuters news agency.
Malinee Sukavrejworakit, the deputy governor of Bangkok, said that four soldiers had been killed in the clashes.
Emergency services reported that at leat 20 soldiers had been injured in a blast, and separately, a grenade exploded near to Abhisit's office in Government House, but there were no initial reports of injuries in that blast.
Aela Cullen, Al Jazeera's correspondent in Bangkok, said reports were emerging of heavy gunfire and teargas use in in the city's famous Khao San Road tourist area.
|Al Jazeera's Wayne Hay reports on the heightening violence in Bangkok on Saturday
More than 500 people, including 64 soldiers and police, were wounded in skirmishes near the Phan Fah bridge and Rajdumnoen Road in Bangkok's old quarter, a protest base near government buildings and the regional UN headquarters.
Hundreds of red shirts, many of whom support Thaksin Shinawatra, the ousted former prime minister, also forced their way into government offices in two northern cities, raising the risk of a larger uprising against Abhisit.
An earlier offensive by troops failed to clear the protesters, who had regrouped by nightfall as troops sent in reinforcements.
After a tense standoff, troops went on a second offensive about 500 metres away at Kok Woa intersection near Khao San Road.
Sansern Kaewkamnerd, an army spokesman, said: "We fear sabotage by the red shirts, so we are reinforcing troops on Rajdumnoen Road and the area to make sure the situation doesn't spiral out of control."
Shopping district siege
Despite the security steps, several thousand red shirts continue to swarm Bangkok's main shopping district, a stretch of upscale shopping plazas and five-star hotels that has been held in a carnival-like atmosphere for a week by the red shirts.
The red shirts used taxis and pickup vans on Saturday to barricade themselves in this area, and expanded their control to include several more blocks.
Riot police who massed at one end retreated after being surrounding by the red shirts.
"We are urging people to go to Pan Fah bridge for reinforcement," Nattawut Saikua, a red shirts leader, said from the shopping district.
"We have a lot of people here. The situation there is much more tense."
Tourists walked among the protesters, many of whom wore surgical masks and carried wet towels expecting riot police to move in with tear gas.
The red shirts vowed that they would besiege governors' offices in the provinces if there was a crackdown on their protest in Bangkok.
Protesters forced their way into the governor's office compound in the northern city of Chiang Mai, and also broke through the gates into the compound of the town hall in Udon Thani in the northeast.
The red shirts' leaders made fiery speeches on a makeshift stage in Chiang Mai, calling the government to dissolve parliament and stop the crackdown. They also rallied outside a government television station.
The government declared a state of emergency in Bangkok and the surrounding areas on Wednesday to control the protests after red shirts broke into the grounds of parliament, forcing some officials to flee by helicopter.
The humiliating failure of security forces to stop protesters from besieging parliament and a satellite station two days later has raised questions over the competence and loyalty of Thailand's armed forces.