Bangkok, Thailand ? Anti-government protesters who have closed down Bangkok’s airports broke through a police cordon meant to shut them off from supplies, raising fears Saturday of widening confrontations in the standoff that has strangled the country’s economy.

About 400 protesters, traveling in a convoy of cars from the occupied international airport, attacked a police checkpoint staffed by more than 100 police. The perimeter, which was put in place earlier in the day, had raised expectations authorities were preparing for a raid to end the four-day-old siege.

But instead, the dramatic 4-minute assault effectively broke the cordon around the airport, which protesters overran Tuesday night as part of their campaign to force Prime Minister Somchai Wongsawat from office.

Bangkok’s domestic airport has also been seized, severing the capital from all commercial air traffic and virtually paralyzing the government.

The closure of the airports has taken a heavy toll on Thailand’s economy and reputation. According to Thai media reports, some 100,000 tourists are stranded, and schedules of airlines around the world have been disrupted.

The protesters, carrying metal rods and some guns, smashed windshields and threw what appeared to be firecrackers at the police. Video footage of the attack appeared to show a protester firing a handgun toward a police van filled with officers.

In another sign the standoff could become more violent, an explosion was reported early Sunday morning inside the Somchai’s office compound, which protesters seized in August and have held ever since. At least 33 people were wounded, including five who were hospitalized, according to a spokesman from the Narentorn Medical Center. He spoke on condition of anonymity in line with policy.