Thailand’s week of antigovernment demonstrations entered a dangerous and volatile phase on Sunday after shootings involving rival political camps left at least four people dead and more than 50 wounded.

 Many areas of Bangkok, the sprawling metropolis that is a major hub of commerce and travel in Southeast Asia, 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 the capital where protests — and violence — have raged.

As protesters traveled through the city by motorcycle and on foot Sunday, vowing to shut down additional government buildings, 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 began arriving in the capital to shore up key government buildings.

Protesters are pursuing the quixotic goal of ridding the country of the influence of Thaksin Shinawatra, a billionaire tycoon and former prime minister whose political party has captured the allegiance of voters in the 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.

Prime Minister Yingluck Shinawatra, who is Mr. Thaksin’s youngest sister, repeatedly said over the weekend that she was open to discussions with protesters but that she would stand firm.

“I will remain here,” she told reporters Saturday, her voice cracking with emotion. “I will not flee anywhere. I may be a woman, but I have the courage to face all possible scenarios.”