Officers fired tear gas and rubber bullets towards protesters trying to force their way into a sports stadium where candidates were gathering to draw lots for their position on polling papers.
The demonstrators, some armed with slingshots, threw rocks and attempted to break through police lines. Inside the stadium, candidates for at least 27 parties took part in the lot-drawing process, which went on unaffected.
Four election commissioners reportedly left the stadium in a helicopter.
Three officers were injured.
It was unclear how many protesters were hurt.
It was the first violent incident in nearly two weeks of daily protests on the streets of Bangkok.
The protesters have been demanding that Prime Minister Yingluck Shinawatra step down since mid-October. They oppose the polls scheduled for February 2 because Yingluck is seen as sure to win them.
Thailand has been racked by political conflict since Yingluck's brother, former Prime Minister Thaksin Shinawatra, was toppled by a 2006 military coup. The protesters accuse Yingluck of being a proxy for Thaksin, who lives in self-imposed exile to avoid jail time for a corruption conviction but still wields influence in the country.
Thaksin or his allies have won every election since 2001 thanks to strong support in the north and northeast.
His supporters say he is disliked by Bangkok's elite because he has shifted power away from the traditional ruling class, which have strong links to the royal family.
On Wednesday, Yingluck announced a proposal for a national reform council to come up with a compromise to the crisis, but it was rejected by the protesters.
They plan more civil disobedience and street protests in a bid to provoke such chaos that Yingluck will be forced to resign as caretaker.
The country's main opposition party, which is allied with the protesters, is boycotting the elections, which Yingluck called early in hopes of giving her a fresh mandate and defusing the crisis.
Yingluck led the country for two years relatively smoothly.
But in October, her government tried to introduce an amnesty law that would have allowed Thaksin to return to the country as a free man, sparking the latest unrest.
