While only a small confrontation between police and protesters angry with Prime Minister Yingluck Shinawatra, the incident came a day after the government extended a special security law for another two months.
Yingluck remains caretaker prime minister after calling a snap election for February 2 in an attempt to deflate weeks of mainly peaceful protests that, at their peak, have drawn 200,000 people onto the streets of Bangkok.
The protesters draw their strength from Bangkok's middle class and elite who dismiss Yingluck as a puppet of her self-exiled elder brother, former premier and telecommunications billionaire Thaksin Shinawatra.
Thaksin and Yingluck have their power base in the rural north and northeast. Their opponents accuse Thaksin of manipulating the poor in those areas with populist policies such as cheap healthcare and easy credit.
About 500 protesters gathered outside a Bangkok gymnasium early on Thursday where Thailand's Election Commission is working through the process of registering candidates for the February election.
Thai media reported that representatives of a number of parties planning to contest the election were inside the building at the time. Calls by Reuters reporters to officials inside could not be connected.
Police warned the protesters not to try to enter the building and then fired several rounds of teargas when demonstrators tried to break down a fence.
Reuters reporters at the scene said police also fired rubber bullets.
The protesters, some of whom had been throwing rocks, soon withdrew from the front of the compound.
Several protesters were affected by teargas but no one was seriously hurt, Reuters witnesses said. The protesters are well prepared for such clashes, the last of which happened about two weeks ago. Many carry goggles and masks to cover their faces and water bottles to wash out their eyes.
The clash came a day after the Thai cabinet voted to extend the Internal Security Act by another two months.
The law, widened last month to cover all of the capital and nearby areas, allows police to ban gatherings, block routes, impose curfews and carry out searches, although such actions have been used sparingly.
Protesters, led by fiery former deputy premier Suthep Thaugsuban, have vowed to disrupt the election and hound Yingluck from office. They want an unelected "people's council" to rule before elections are called.
The election has been made more uncertain by a boycott by the main opposition Democrat Party, which draws its support from Bangkok and the south, the same base as Suthep's group.
Also on Wednesday, Yingluck proposed the creation of an independent reform council to run alongside the elected government, an apparent attempt at compromise that was immediately rejected by the protesters.
Yingluck has not been in the capital for most of the past week, choosing instead to shore up her support in her power base to the north, and will not return to Bangkok until the New Year.
Her Puea Thai Party is almost certain to win the election, just as Thaksin's populist political juggernaut has won every vote since 2001. That run of success has come despite violent street protests and judicial and military intervention around previous polls.
Thaksin was overthrown in a 2006 coup and has lived in self-imposed exile since 2008, when he was sentenced to two years in jail for graft charges he says were politically motivated.
The first two years of Yingluck's government had been relatively smooth, until her party miscalculated in November and tried to push an amnesty bill through parliament that would have allowed her brother to return home a free man.
