Protesters targeted Thailand's elections watchdog on Monday, at one point storming its office, while ignoring orders to leave Bangkok's main shopping district for a third day in an increasing bold rally to force elections.

The protesters in the election office later dispersed, but tens of thousands of the red-shirted demonstrators, supporters of ousted Prime Minister Thaksin Shinawatra, remained encamped in a district of luxury hotels and department stores.

Thousands of others moved out across the city on trucks, waving red flags.

The "red shirts" accuse the Election Commission of stalling in an investigation of alleged irregularities by Abhisit's ruling Democrat party and set a deadline for the Election Commissioner to answer their questions in person after thousands of protesters gathered outside the building.

When he failed to show, hundreds swarmed into a ground-floor hallway but were blocked by police and repelled into compound grounds. They eventually left after police gave assurances the Election Commission would hear the case on April 20.

The storming of the building came hours after the protesters threatened to expand their increasingly confrontational rally, which Bank of Thailand Governor Tarisa Watanagase said could hurt the economy if it continues.

Despite threats they could face a year in jail, the protesters have said they had no plans to leave the downtown area, forcing retailers to shut their doors.

Many tore up fliers ordering them to vacate.

Abhisit has called the rally "unlawful", but there has been no sign security forces would disperse the mostly rural and working class "red shirts", who say they will not leave until parliament is dissolved.

The government filed a court order on Monday to disperse the crowd and planned to file another to arrest leaders of the rally, which it says violates Thailand's tough Internal Security Act imposed last month to maintain order during the protests.

But the protesters remained defiant.

"We won't leave. We have sent our lawyer to the court and will submit an objection immediately if the court issues any order to force us to leave," said Nattawut Saikua, a protest leader.

The "red shirts" now occupy three areas: the shopping district, Phan Fah Bridge in Bangkok's old quarter and the Election Commission's office.

Agencies