Troops fired rubber bullets at supporters of the United Front for Democracy against Dictatorship (UDD) and live shots into the air to stop a red-shirt convoy moving to a fresh-food market on Wednesday.
A soldier was killed and 18 people wounded in the afternoon clashes between troops and supporters of the UDD, the Erawan Emergency Centre said.
The soldier was shot in the head and during the tense confrontation when troops blocked the protesters from going to Talad Thai to campaign for the anti-government rally. It was reported they also intended to establish a roadblock there to prevent police and troop reinforcements entering Bangkok.
It was the 27th fatality since UDD supporters set up their protest camp in Bangkok streets, demanding the end of the Abhisit Vejjajiva government.
A Bangkok Post photographer reported that several protesters were arrested during the clash.
There was a report that prominent UDD member Kwanchai Praipana, who is named in an arrest warrant, was among them. But he appeared on the protesters' main stage after returning from the mobile rally in the evening to tell the crowd about the incident.
The clash occurred near the National Memorial, close to Don Muaeng airport, on Vibhavadi Rangsit Road.
Soldiers fired rubber bullets and tear gas at the estimated 2,000 protesters travelling on motorcycles and in pick-up trucks as they attempted to breach wire barricades set up near the National Memorial in Pathum Thani's Khu Khot district about 2pm, according to television reports. The convoy was on its way to Talad Thai fresh market.
The protesters retreated, but some retaliated by hurling stones and large home-made firecrackers at the soldiers. The clash came to an abrupt end as heavy rain poured down.
On hearing of incident, UDD leader Jatuporn Prompan ordered the protesters to immediately regroup at their main stronghold in Ratchaprasong commercial district.
The clashes brought traffic on Vibhavadi Road to a standstill.
The clashes took place a few hours after UDD protesters left their camp in Ratchaprasong commercial district, saying they planned to explain the facts about their protest to the public at Talad Thai and try to persuade them to join their anti-government rallies.
Mr Kwanchai was assigned to lead the convoy.
"We want to thank them and give them encouragement for helping us the other day and also talk to our supporters and friends at the market," Mr Kwanchai, also known as Isan Rambo, said before the convoy left Ratchaprasong.
"If the troops try to stop us, we will break through their cordons and checkpoints," he said, adding the convoy would return to their base after a few hours.
Authorities had earlier warned the protesters they would take decisive action if they left their rally site and mobilised to other areas.
On Tuesday, Prime Minister Abhisit Vejjajiva warned that the government would do all it could to thwart the UDD's plan to tour the capital.
Deputy Prime Minister Suthep Thaugsuban said authorities had been patient for two months and warned they would use decisive measures available to them under the law from now on.
But UDD leader Natthawut Saikua replied by saying that the UDD is not afraid if authorities try to stop them.
"If the military thinks it is necessary to use force to block us, it's all right," he said. "We are not afraid."
