This is the second time in the month-long protest campaign that police used tear gas in Thailand's worst political violence in years. Fifteen protesters were injured in the tear gas and firing, Nation Online said adding that three police officers had also been hospitalised.
Police fired several rounds of tear gas around 7:20 am after protesters led by the Students and People Network for Thailand's Reform broke the padlock of a gate to enter the stadium.
The opposition Democrat Party has said it will boycott the polls scheduled for February 2. The clash broke out as representatives of 27 political parties arrived at the stadium for the draw of party-list numbers for the elections.
Police closed all exits and parked police vehicles in front of the gates to prevent protesters from entering. Meanwhile, the Centre for Administration of Peace and Order (CAPO) defended the police's use of tear gas against anti-government protesters.
Deputy Prime Minister and Foreign Minister Surapong Tovichakchaikul, who also heads CAPO, said police had used full restraint in dealing with the protesters who had the right to demonstrate their opposition.
Fifteen protesters were injured in the tear gas and firing.
However, the government and police could no longer tolerate their illegal actions as the country had laws that should be respected by all, he said.
He insisted that the handling of the situation met international standards. However, Nation Online reported that police gave only a single warning to the protesters before firing tear gas.
The demonstrators, some armed with sling shots, in retaliation threw rocks at police. The anti-government protesters have been demanding that Prime Minister Yingluck Shinawatra quit, since mid-October.
The protests began after Yingluck's government tried to introduce a contentious amnesty bill which could have paved the way for the return of her brother, controversial former premier Thaksin Shinawatra, currently in self-imposed exile in Dubai after being overthrown in a coup in 2006.