BANGKOK — The week-old campaign of antigovernment protests in Thailand entered a dangerous new phase on Sunday after shootings involving rival political camps left at least three people dead and more than 110 wounded in Bangkok.

Many areas of this sprawling capital remained unaffected by the demonstrations. But the shootings and the increasingly provocative moves by protesters spread fears that unrest could move beyond the pockets of Bangkok where demonstrations — and violence — have raged.

As protesters seeking to take over the prime minister’s office clashed with riot police officers, Bangkok’s largest shopping malls, which normally teem with visitors on weekends, hastily announced that they were closing their doors for the day.

Nearly 3,000 soldiers arrived in the capital to shore up critical government buildings.

The protesters have banded together in a seemingly quixotic attempt to end the influence of Thaksin Shinawatra, a billionaire tycoon and former prime minister whose political party has captured the allegiance of voters in the Thai countryside, winning every election since 2001. The protesters say they are frustrated with the dominance of Mr. Thaksin and are disillusioned with the current democratic system. They have proposed an alternative to the country’s democracy — an ill-defined people’s council made up of representatives from many professions.