BANGKOK — Thai troops fired rifles and threw tear gas at a crowd of anti-government protesters riding motorbikes down a busy expressway Wednesday, blocking their effort to take the demonstrations that have paralyzed central Bangkok into the suburbs.

The hours-long confrontation killed one soldier – apparently from “friendly fire” – and wounded 18 other people as it transformed the suburban streets into a battle zone. Heavily armed troops took cover behind terrified commuters’ cars and one driver clasped her hands in prayer as the soldiers wove their way through traffic.

Security officials suggested the possibility of an escalation in the violence. In a television broadcast Wednesday night, they displayed 62 grenades they said had been found in a bag dropped by a motorcyclist who fled a police checkpoint on a road leading to the site of the clash.

The bloodshed was the first violent confrontation in nearly a week, and protest leaders accused the government of leading the nation to the brink of civil war.

“Our side is running everything in order to create peace, but the government is trying to push war. And you know if (they) push war, civil war is coming,” said Weng Tojirakarn, a leader of the Red Shirt protesters.

The Red Shirts have turned parts of Bangkok’s commercial heart into a protest camp in their campaign to bring down a government they view as illegitimate.

Government officials say they want to end the standoff peacefully but cannot tolerate the protest enclave, which has forced the closure of some of the city’s ritziest malls and hotels and cost businesses millions of dollars a day. The unrest has devastated the vital tourist industry, and several countries have warned their citizens against travel to Bangkok, or even Thailand as a whole.

Authorities have so far resisted breaking through the Red Shirts’ barricades and clearing them out of the city, an operation that would almost certainly cause more casualties.

Yet patience appeared to be running out in the seven-week standoff that has killed at least 27 people and wounded nearly 1,000.

In an interview broadcast Wednesday on BBC World News, Prime Minister Abhisit Vejjajiva said he was working to quickly restore order.

“But at the same time we have to be aware of the need to make sure that there will be minimum losses and to make sure that we comply with international standards and respect the basic rights of people, including those of the protesters,” he said.

The Red Shirts get much of their support from poor, rural provinces and see the British-educated prime minister as a symbol of an urban elite uncaring about their plight. But their cause has also drawn support from the urban working class and people opposed to the influence of the military in politics.