At the Chaeng Watthana stage, protesters built a bunker with more than 200 tyres and covered the site with large nets to protect protesters from explosive devices that could be hurled at them.
At the Lat Phrao stage, PDRC co-leader Issara Somchai ordered security guards to search the belongings of people entering the site to make sure they were not carrying weapons.
More than 500 guards will also patrol the area during the day and at night.
"We believe our opponents have disguised themselves as protesters and mingled with us. If we arrange more patrols, we will be able to prevent them from committing violence," he said.
The move came after a 53-year-old security guard was shot and severely injured at the rally site on Saturday.
Mr Issara said he heard three to four gunshots early yesterday at the entrance of the protest site near the TMB Bank building on Phahon Yothin Road. No one was injured.
At the Victory Monument, protest leader Thaworn Senneam instructed his staff to install almost 30 surveillance cameras around the site to collect evidence in the event of attacks.
Security guards there admitted they had covered 16 surveillance cameras around the protest site with black bags as they had feared state authorities would use the cameras to spy on the protesters' movements.
The cameras belong to City Hall and business operators.
Bangkok governor MR Sukhumbhand Paribatra on Monday said he would work with the protest leaders to remove the covers to allow the cameras to function normally.
Sanya Chantharut, an adviser to the Bangkok governor, visited the rally site yesterday to discuss the issue. He said City Hall's surveillance cameras were installed for the benefit of the public.
Police set up four checkpoints around the Victory Monument rally site with five police stationed at each checkpoint to help provide security to the protesters, said Chusak Techarakpong, deputy commander of the Metropolitan Police Region 1.
The checkpoints will be operated from 8am to 8pm, he said.
Meanwhile, Ratchaprasong businessman Somboon Namtipchantacharoen yesterday accused PDRC guards of assaulting him while he distributed leaflets on Sunday night and Monday morning calling on the PDRC to call off their rallies.
He said he and other business operators had lost money since the PDRC launched its Bangkok shutdown operation in which the protesters rally at key locations, including at Ratchaprasong.
Mr Somboon, the owner of the Isotip eyeglasses shop, will talk to the media today about the impacts of the protest.