Officers fired tear gas and rubber bullets toward 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 sling shots, threw rocks and attempted to break through police lines and one policeman was killed.
Inside the stadium, candidates for at least 27 parties took part in the lot-drawing process, which apparently went on unaffected despite the turmoil outside the gates.
Police Colonel Anucha Romyanan had urged the demonstrators to assemble peacefully and said "attempts are being made to escalate the political situation by causing violence".
Thailand's election commission called on the government to delay the polls scheduled for February 2 in the wake of yesterday's fresh violence.
The commission said it was urging the government to consider postponing the elections, citing the lack of peace between the government and protesters.
But the government later rejected the call from the election commission and insisted the vote would go ahead as planned.
Deputy Prime Minister Pongthep Thepkanchana said in a televised address: "There is no law allowing the government to delay the election"
It was unclear how many protesters were hurt in yesterday's clashes which were contained to the area around the stadium. It was the first violent incident in nearly two weeks of daily protests on the streets of Bangkok.
The protesters have been demanding prime minister Yingluck Shinawatra step down since mid-October and street unrest has occasionally broken out.
They oppose the polls scheduled for February because Ms Yingluck is seen as sure to win them.
Police have largely shown restraint and have made no move to arrest opposition leader Suthep Thaugsuban who is demanding the country be led by an unelected council until reforms can be implemented.
Thailand has been wracked by political conflict since Ms Yingluck's brother, former prime minister Thaksin Shinawatra, was toppled by a 2006 military coup.
The protesters accuse Ms Yingluck of being a proxy for her brother, who lives in self-imposed exile to avoid jail time for a corruption conviction but still wields influence in the country and is said to conduct cabinet meetings via video conferencing.
He or his allies have won every election since 2001 thanks to strong support in the north of the country. 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.
Ms Yingluck earlier this week 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 now plan more civil disobedience and street protests in a bid to provoke such chaos that she will be forced to resign.
The country's main opposition party, which is allied with the protesters, is boycotting the elections, which Ms Yingluck called early in hopes of giving her a fresh mandate and defusing the crisis.
She led the country relatively smoothly for two years but in October tried to introduce an amnesty that would have let her brother return to the country a free man, sparking the latest unrest.
National Security Council head Paradorn Pattanathabutr said the police response during yesterday's demonstration did not mark a change of policy.
He said: "We have warned them and informed them every time before firing teargas."