The clashes began after dawn when police sought to clear a path through some 5,000 members of the People's Alliance for Democracy demonstrating outside parliament since Monday in a bid to prevent cabinet ministers from the opening session.
More than 100 people were wounded in the clashes with police, the Associated Press reported. Rioting protesters also set fire to parked cars, trucks and vans.
One protester lost a foot and another had his leg severed by exploding gas canisters, prompting Deputy Prime Minister Chavalit Yongchaiyudh to take responsibility and quit. He said he had asked police to exercise restraint, according to Reuters.
"Since this action did not achieve what I planned, I want to show my responsibility for this operation," Chavalit said in his resignation letter.
Army commander Anupong Paochinda said police asked for military help and denied rumors of a fresh coup, two years after the military ousted Prime Minister Thaksin Shinawatra over accusations of corruption.
"People should not panic. Soldiers will not launch a coup since it will not be good for the country," he told reporters.
The military has staged 18 coups since the country became a constitutional monarchy in 1932.
Separately, an unidentified person was killed when a Jeep SUV exploded near the headquarters of the Chart Thai Party, a member of the six-party coalition government of Prime Minister Somchai Wongsawat.
The chaotic protests are the latest twist in a political crisis that has gripped Thailand for six weeks.
Prime Minister Somchai was sworn in as prime minister on Sept. 25 but has been forced to run the government from a makeshift office at Bangkok's Don Muang airport after protesters occupied the grounds of the prime minister's offices.
Opposition groups say Somchai, Thaksin's brother-in-law, is a proxy for the ousted prime minister. The PAD claims Thai democracy has been hampered by billionaire Thaksin, who now lives in exile, and his allies, who easily won the last three elections.
The anti-government group, believed to be backed by an alliance of businesses, academics and activists, has vowed to keep campaigning until Somchai dissolves parliament, according to Reuters.
Media reports describe Tuesday's parliamentary protest as well organized with supplies of water and food on hand for demonstrators along with masks to protect against tear gas.
"Overthrow the Thaksin regime. Together we win or lose. We will know it today. We won't give up," PAD leader Anchalee Paireerak said.
Somchai was able to open Tuesday's parliamentary session after a 90-minute delay but as he spoke chaos grew outside the building.
"This government is determined to tackle economic problems and to listen to all sides to find a solution to end the crisis," Somchai said.
He reportedly slipped out through a back gate to a waiting helicopter, which whisked him out of the parliament grounds.
Somchai's predecessor, Samak Sundaravej, was forced to step down Sept. 9 after a court found him guilty of conflict of interest for a paid appearance on a cooking show.