Forty-six people were injured, including two seriously, said Petpong Kumtonkitjakarn of the Erawan Medical Center.
"One of them lost his leg, another was hit with shrapnel in the chest," he said.
The morning clashes came just hours before Prime Minister Somchai Wongsawat, who was sworn in Sept. 25, was scheduled to deliver his government's policy statement to lawmakers.
Protesters from the People's Alliance for Democracy — which is seeking the government's resignation and a major overhaul of the electoral system — have occupied the grounds of the prime minister's office for six weeks. Late Monday, they expanded their protest by marching to Parliament, vowing to block lawmakers from entering the building.
After the morning clashes, thousands of protesters regrouped in front of Parliament where speakers addressed the crowd from a makeshift stage.
"Fight with us in protecting this country! Stay with us here until we have our victory," a speaker told the cheering crowd.
The action by alliance activists came in response to the recent arrests of two of its leaders, and seemed intended to spark a confrontation to revive its flagging movement.
The alliance says Somchai is a proxy for ex-Prime Minister Thaksin Shinawatra, who was ousted in 2006 by military leaders who accused him of corruption and who now resides in exile. Somchai is a brother-in-law of Thaksin.
When protesters originally took over the grounds of Government House on Aug. 26, their intention was to oust then Prime Minister Samak Sundaravej — whom they also accused of being a puppet of Thaksin. They later said they also opposed his successor, Somchai.
Samak was dismissed from office on Sept. 9 by a court decision that found him guilty on a conflict of interest charge. He had accepted pay for hosting a TV show while in office.
At the nearby Government House, protest leaders called for supporters to join their ranks.
"Brothers and sisters, please come out and help us," said Somsak Kosaisuk, one of nine protest leaders. "We were protesting peacefully. I urge you to come out to join us in our fight against this illegitimate government."
"We will not stop," Somsak said. "We will fight until we have our victory!"
The alliance claims Thailand's rural majority — who gave strong election victories to the ruling People's Power Party — is too poorly educated to responsibly choose their representatives and says they are susceptible to vote buying.
The protest group wants the country to abandon the system of one-man, one-vote, and instead have a mixed system in which some representatives are chosen by certain professions and social groups. They have not explained how exactly such a system would work or what would make it less susceptible to manipulation.
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