Live ammunition was found at one site of confrontations between protesters and the authorities.
The worst of the skirmishes took place at Phon 1, Panich Chayakarn and Karn Ruen intersections, and Orathai and Chamai Maruchet bridges.
One faction of protesters tried to seize the Metropolitan Police Bureau (MPB) near Phon 1, while others attempted to storm Government House and parliament. The three locations were the prime targets of the demonstrators, many moving from their bases at Democracy Monument and Nang Loeng.
Police fired tear gas, sprayed water cannon and, for the first time, used rubber bullets to deter the protesters. At least one real bullet was found at a rally site. A young male protester, believed to be about 25, was reportedly shot in the leg and taken to Ramathibodi Hospital.
Protesters accused police of firing real bullets to break up the protests.
Anti-government protesters clash with police officers in different areas of Bangkok on Monday. (Bangkok Post photos)
But Pol Maj Gen Piya Uthayo, spokesman for the Centre for the Administration of Peace and Order (Capo), denied that anything other than tear gas, rubber bullets and water cannon were used.
Police are investigating the case of the protester injured by the live bullet, the spokesman added.
The Civic Movement for Democracy led by Suthep Thaugsuban on Sunday stepped up its rallies to oust Prime Minister Yingluck Shinawatra and eventually end the Thaksin regime, after clashes between Ramkhamhaeng University students and red-shirts on Saturday night.
Erawan Emergency Centre of the Bangkok Metropolitan Administration reported that 58 protesters had been injured since Sunday. The death toll still stands at three.
Capo reported that 37 police had been injured in the clashes.
The number of casualties is expected to rise as the violence still continues. Unconfirmed reports said the injured had now reached 94.
For the first time since the rallies begun, medics from the Army were seen on Ratchadamnoen Nok Avenue offering medical aid to protesters hurt by tear gas.
Protesters trying to break into police-controlled areas around Government House and parliament were unable to move forward, but the group at Phon 1 edged close to MPB since police decided to open all but last barricades protecting the city's main police office. But protesters decided not to push on after facing large rounds of tear gas.
Demonstrators deployed various tactics to counter police tear gas. Many threw canisters back at officers and large fans were used to blow the gas back in the direction where the police were stationed.
Anti-government protesters said they plan to spray faeces at police who use tear gas to prevent them from occupying government offices.
Chumpol Jullasai, a core member of the anti-government group, announced the plan from the stage at Ratchadamnoen main protest site on Monday.
"We'll use a biological weapon to counter police who use tear gas on us," he said.
Protest co-leader Sathit Wongnongtoey told the protesters at Ratchadamnoen that the group aimed to force the "Thaksin regime" to step down before Thursday.
"We don't want the current administration to be the ones who organise His Majesty the King's birthday ceremony (on Thursday Dec 5)," said the former Democrat Party MP.
He said the anti-government group would separately organise a birthday celebration for His Majesty if they did not meet their objective and the celebration would be grander than the government's.
Mr Sathit insisted a House dissolution is not a solution to the problems, because it would not cleanse Thailand of those serving former premier Thaksin Shinawatra. The only way out is to strip the state of its power so the country can be reformed as a whole, he said.
"The reform model has already been drafted by several lawyers, but this is not the time to show it to the public," he added.