Thai troops fired rubber bullets and tear gas at thousands of demonstrators in a second offensive in Bangkok on Saturday, the biggest confrontation of month-long street protests for new elections.

Protesters fought back, hurling petrol bombs at troops.

At least 171 people, including 64 soldiers and police, have been 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 U.N. headquarters.

Hundreds of "red shirt" protesters also forced their way into government offices in two northern cities, raising the risk of a larger uprising against Prime Minister Abhisit Vejjajiva.

An earlier offensive by troops firing rubber bullets and smoke bombs had 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 (1,600 ft) away at Kok Woa intersection which leads to Bangkok's famous Khao San Road tourist area.

"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," army spokesman Sansern Kaewkamnerd told Reuters.

Tens of thousands of protesters also remained in Bangkok's main shopping district, a stretch of upscale shopping plazas and five-star hotels and held for a week by the red shirts in a carnival-like atmosphere.

The red shirts used taxies and pickup-trucks to barricade themselves in that a

rea, and expanded their control to include several more blocks. Hundreds of riot police who massed at one end retreated after being surrounding by red shirts.

"We are urging people to go to Pan Fah bridge for reinforcement," Nattawut Saikua, a red-shirt leader told Reuters from the shopping district. "We have a lot of people here. The situation there is much more tense."

In the shopping district, tourists walked among the protesters, many of whom wore surgical masks and carried wet towels expecting riot police to move in with tear gas.

PROTESTS IN NORTHERN CITIES

The red shirts had vowed that they would besiege governors' offices in the provinces if there was a crackdown on their protest in the capital.

Hundreds of protesters forced their way into the governor's office compound in the northern city of Chiang Mai and hundreds broke through the gates into the compound of the town hall in Udon Thani in the northeast.

Protest leaders made fiery speeches on a make-shift stage in Chiang Mai, calling the government to dissolve parliament and stop the crackdown. They also rallied outside a government television station.

About 50 police were at the scene in Chiang Mai, hometown of former Prime Minister Thaksin Shinawatra, the figurehead of the red shirts who was ousted in a military coup in 2006, but did not intervene.

The government declared a state of emergency in Bangkok 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 in a month of increasingly bold street protests for elections.

(Writing by Jason Szep; Additional reporting by Damir Sagolj; Editing by Nick Macfie)