Three people were shot, one of whom was seriously wounded, while others mostly suffered from tear gas exposure, the Public Health Ministry reported.
The skirmishes at Pol 1 intersection and Chamai Maruchet bridge, where the clashes broke out on Sunday, resumed in the morning when thousands of protesters tried to break through police barricades to get into the Metropolitan Police Bureau (MPB) and Government House.
Clashes also erupted at Orathai bridge near Government House and at Karn Ruan intersection near parliament.
At all sites, protesters tried to remove giant concrete barriers and cut barbed-wire fences, prompting police to use tear gas and water cannons.
The situation was most tense at Chamai Maruchet bridge where a Bangkok Post reporter saw at least four people, including a Daily News photographer, get shot.
An Al Jazeera car was also shot at, but their crew was not injured. Another Thai reporter sustained head injuries from a hard object hurled at him.
A field hospital was set up at the nearby Royal Turf Club in the Nang Loeng area where hundreds of protesters who were injured during the Chamai Maruchet and Orathai clashes streamed in to seek first aid. They mostly suffered eye and skin irritation and bruises.
Ambulances were stationed at the site to transfer serious cases to hospitals.
About 7pm, protesters tried to remove concrete barriers at Chamai Maruchet bridge with a bulldozer, prompting police to fire tear gas at them. The clashes were continuing as of press time.
At Orathai bridge, another group of protesters, mostly vocational students, attacked police guarding Government House with rocks and water bottles. Police fired back with tear gas. Protesters failed to break the police line and retreated to Nang Loeng in the evening.
At Pol 1 intersection, protesters yesterday successfully removed three of four layers of concrete barriers, allowing them to move closer to the MPB. As the clash escalated, deputy Bangkok governor Asawin Kwanmuang arrived at the MPB and held talks with deputy police chief Worapong Chewpreecha.
Following the talks, police said they would move back five metres and the protesters agreed to cease their attacks.
At Karn Ruan intersection, at least 10 protesters were injured during the day-long clash as they tried to break through police barricades to proceed to parliament.
Apart from deploying a bulldozer to remove concrete barriers, the protesters also used giant fans to blow back tear gas police fired at them.
Sacks were delivered to the clash sites and protesters soaked them in water and used them to cover tear gas canisters hurled at them. The sodden sacks helped prevent the tear gas spreading.
Earlier in the day, protest leader Chumpol Jullasai announced at Democracy Monument that the protesters planned to spray faeces from faecal suction trucks at police who use tear gas to prevent them from occupying government offices.
However, there were no reports of protesters using faeces yesterday.
The Public Health Ministry reported that a total of 221 people were injured and three killed in the unrest in Bangkok from Saturday until 4pm yesterday.