At least one person has been killed and three wounded by gunfire after clashes broke out between rival protesters in the Thai capital Bangkok.
People heading to a pro-government rally were attacked by students, and later shots were fired.
Saturday is the seventh day of protests aiming to unseat the government of Prime Minister Yingluck Shinawatra.
Protesters claim her government is controlled by her brother, exiled former leader Thaksin Shinawatra.
He was ousted in a coup following protests in 2006 and now lives in self-imposed exile overseas.
He is one of the most polarising figures in Thai politics - he remains popular with many rural voters, while his opponents tend to be urban and middle class voters.
On Saturday, what had been largely peaceful demonstrations turned violent outside a stadium where a pro-government rally has been confined during this week to avoid clashes with the anti-government protests moving around the city, the BBC's Jonathan Head reports from Bangkok.
A group of students attacked vehicles bringing government supporters to the stadium - windows were smashed, and some minor injuries reported.
Later, shots were fired, but it is not clear yet by whom.
Police reinforcements were sent to the area and roads blocked, but skirmishes between the two sides continued for several hours.
Police have called for military backup to reinforce security in the city.
National police spokesman Piya Utayo said on Thai television that some 2,730 military personnel from the army, navy and air force will be deployed, AFP reports.
The government has been reluctant to risk deploying the military, which ousted Ms. Yingluck's brother in a coup seven years ago, but may no longer have a choice, our correspondent reports.
Tension is now rising in Bangkok as the anti-government movement prepares for what it calls a "people's revolt" - a mass occupation of government buildings all over Bangkok.
There is a high risk of greater violence if more supporters of Ms Yingluck - whose party enjoys strong backing outside Bangkok - try to come to the capital, our correspondent adds.
On Friday Ms Yingluck ruled out early elections, telling the BBC that the country was not calm enough for polls.
She repeated her call for negotiations to resolve the crisis.
Demonstrators have been surrounding and occupying official buildings this week in an attempt to disrupt the government.
On Friday at least 1,000 protesters forced their way into the army headquarters compound, but did not enter any buildings.
During the demonstrations, which had been largely peaceful until Saturday, participants have cut the electricity supply to the national police headquarters and forced the evacuation of Thailand's top crime-fighting agency.
Ms Yingluck has invoked special powers allowing curfews and road closures, and police have also ordered the arrest of protest leader Suthep Thaugsuban - but so far no move has been made to detain him.
The country is facing its largest protests since 2010, when thousands of "red-shirt" Thaksin supporters occupied key parts of the capital. More than 90 people, mostly civilian protesters, died over the course of the two-month sit-in.
The proposed passage of a controversial political amnesty bill, which critics said would have facilitated the return of Thaksin without having to serve jail, reignited simmering political tensions.
The Senate rejected the bill, which sought to cover offences committed during the upheaval after Thaksin was removed from office.
Ms Yingluck said she accepted the vote and would not resubmit the legislation.