Colonel Sansern Kaewkamnerd told local radio that troops fired into the air first in response to tear gas and smoke bombs thrown at them by protesters, but then fired real bullets.
Local television footage showed injured people being taken away as soldiers secured the area that is close to the largest military base in the city.
Call for revolution
The confrontation is the first serious clash between the demonstrators and government forces after weeks of protests by the so-called Red Shirts who support Thaksin Shinawatra, the prime minister ousted in a military coup in 2006.
Thaksin, who continues to wield considerable political influence in the country despite being mostly in exile since September 2006, called on his supporters to overthrow the government on Sunday.
He said he may return to Thailand and re-enter politics if the government moved to crack down on the protests.
Thaksin's call for revolution came after Abhisit Vejjajiva, the prime minister who has been in office for about four months, declared a state of emergency in the city on Sunday following protests on Saturday that forced the cancellation of a regional summit in the nearby town of Pattaya.
Abhisit had warned protesters to go home on Sunday or face tough action as tanks and troops took up positions in the capital.
"The government has tried all along to avoid violence, but the protest has developed and they have used actions incompatible with the constitution," the prime minister said.
Al Jazeera's Tony Cheng, reporting from Bangkok, said on Sunday that Abhisit had appeared to be losing control of the police, with some policemen seen to be putting on red shirts in support of Thaksin and joining the protest rallies.
In further signs that the government was losing control on Sunday, demonstrators swarmed around two armored vehicles outside a luxury shopping mall in downtown Bangkok, before directing the soldiers to drive the vehicles back to a military camp.
Outside the interior ministry, Abhisit's car was attacked with poles, a ladder and flower pots.
At least six people were injured in that incident, including two security guards for the prime minister, and one of Abhisit's senior political aides and his driver, as police nearby did nothing.