At one point in the chaos, security forces fired on a group of troops riding toward them on motorbikes in what appeared to be an accident, although some members of the security forces have been accused of siding with the protesters. At least four motorbikes crashed and one soldier was carried away on a stretcher, bleeding profusely from the head.
The Red Shirts, who have paralyzed parts of the capital for weeks with protests to bring down a government they view as illegitimate, had announced they were widening their demonstrations and dared the military to stop them.
Hundreds headed on motorbikes and pickup trucks to rally in a suburb, but the major road connecting Bangkok with its northern suburbs was blocked by government troops and razor wire.
Riot squads fired into the air to push back protesters, but several tried to remove the razor wire, prompting troops to level their rifles and shoot directly at protesters who fled into oncoming traffic. Police huddled behind riot shields, while soldiers wielding rifles took up positions behind concrete pylons.
According to the government's Erawan emergency center, a soldier was killed and 18 people wounded during the clash. Two of the wounded were soldiers, and the others were protesters.
The confrontation ended Wednesday evening, and the protesters headed back to their enclave.
But the shooting raised concerns that more violence was to come after government warnings that patience was running out in the seven-week standoff.
Army spokesman Sansern Kaewkamnerd said troops were told to use rubber bullets but also had authority to use live ammunition for self-defense.
"We brought force out to stop them. At this point, society finds it unacceptable to have protesters traveling in a motorcade like this," Sansern said. "We try our best to prevent losses."
The demonstration appeared intended to provoke the government, which has repeatedly said it will not tolerate protests beyond the Red Shirts' enclave in the city. It was the first confrontation since five grenades were fired near a gathering of counter-protesters Thursday, killing one person.
At least 26 people have been killed and nearly 1,000 wounded since protesters began occupying parts of Bangkok in mid-March, barricading major roads with tires and bamboo sticks and forcing the closure of some of the city's ritziest malls and hotels. The unrest has devastated the tourist industry, and several countries have warned their citizens against travel to Bangkok, or even Thailand as a whole.
On Wednesday, the Red Shirts, who are demanding the dissolution of Parliament, said they were sending a team of activists to drum up support at an outdoor market north of the capital. They have set up barricades on roads leading to the capital in recent days to prevent police reinforcements from being sent to assist in a possible crackdown.
"We are going to send (protesters) out of the rally site," a Red Shirt leader, Nattawut Saikua, said. "If the military thinks it is necessary to use force to block us, it's all right. ... We are not afraid."
Prime Minister Abhisit Vejjajiva has repeatedly said he hopes to resolve the conflict peacefully but has yet to outline a clear plan out of the crisis after calling off negotiations with the protesters. They are drawn mostly from poor, rural provinces and see the premier as a symbol of an elite impervious to their plight.
Abhisit's government has been criticized for excessive leniency and standing aside as protesters occupied more than 1 square mile (3 square kilometers) in the heart of Bangkok.
The government has accused the protesters of trying to undermine the country's revered king. Such a charge, which protest leaders strongly deny, could weaken their support.
Deputy Prime Minister Suthep Thaugsuban said the government was investigating opposition politicians, academics and media it believes are involved in an anti-monarchy movement, the newspaper Matichon reported. Thailand's lese majeste law mandates a jail term of up to 15 years for anyone who "defames, insults, or threatens" the royal family.
The king, long seen as the best hope for a solution to the crisis, spoke publicly for the first time this week, but only obliquely addressed the unrest.
The Red Shirts' ranks are largely drawn from supporters of former Prime Minister Thaksin Shinawatra and pro-democracy activists who opposed the military coup that ousted him in 2006 on corruption allegations. The group believes that Abhisit's government - backed by the urban elite - is illegitimate, having been helped into power by the country's powerful military.
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Associated Press writers Ravi Nessman and Jocelyn Gecker contributed to this report.
