A series of minor attacks on Tuesday night has fuelled fears of escalating violence as the conflict between the government and its opponents shows no sign of abating.
Both sides blamed each other for the attacks.
Two people were injured in a bomb and gun attack near the Chalerm La 56 bridge, not far from the Pathumwan intersection rally site, around midnight on Tuesday.
It followed a bomb attack on the housing compound of Democrat Party leader Abhisit Vejjajiva a short time earlier.
A bus taking protesters from the South to a rally site at Nang Loeng was also set on fire early yesterday.
The attacks prompted army chief Prayuth Chan-ocha to send soldiers to the Pathumwan intersection protest site to tighten security.
People's Democratic Reform Committee (PDRC) core leader Thaworn Senneam, who is in charge of the rally stage at the Victory Monument, is liaising with the Metropolitan Police Bureau (MPB) to work out security measures at the seven protest sites.
The attack on Mr Abhisit's home on Sukhumvit Soi 31 occurred about 11.30pm. No one was injured.
The explosion damaged the roof of one of the three houses inside the compound.
The damaged home belongs to Mr Abhisit's father, Atthasit, who shares the compound, Thong Lor police said.
Mr Abhisit and his family were not at home.
In the attack at Chalerm La 56 bridge, garbage collector Thossapol Ngamlamai, 35, sustained shrapnel wounds to the head and leg while Amphai Charoenrit, 55, from Surat Thani, suffered a gunshot wound to her right arm.
Both were sent to Chulalongkorn Hospital.
A bullet hole was also found in a pickup truck parked opposite the Asia Hotel.
The arson attack on the bus occurred at the Nang Loeng racecourse in Dusit district about 2am.
It took firefighters about 15 minutes to bring the flames under control. No injuries were reported. The bus was slightly damaged.
Driver Ampai Saengpratheep said the bus had brought protesters from the southern province of Songkhla to the rally site at the Chamai Maruechet bridge on Sunday.
The area is controlled by the Network of Students and People for Reform of Thailand (NSPRT).
Mr Ampai said he had parked the bus near the Nang Loeng racecourse shortly before the attack.
Centre for the Administration of Peace and Order deputy spokesman Anucha Ramayanant yesterday said that recent episodes of violence had been instigated mainly at night.
A military source said efforts are being made to stir up violence to deter people from joining the anti-government rallies.
There are also concerns that attacks on demonstrators could become more violent, the source added.
The military is wary of a possible surge in violence, which could prompt it to intervene, the source said.
A well-placed source at the MPB said police are trying to avoid heavy-handed measures against the protesters to prevent any violent confrontations.
Another source at the Royal Thai Police said he could not tell who would gain the upper hand in the political struggle. All the police can do is to prevent any violence.
"Police are now on the horns of a dilemma," he said.
Pheu Thai Party list-candidate and red-shirt co-leader Cherdchai Tantisirin claimed the violence stemmed from the anti-government groups themselves.
He believed the PDRC and NSPRT were trying to incite violence so the military would be forced to step in and stage a coup.
He said violence may also be perpetrated by people who are angered by the protesters' road blockades.
Mysterious "third-hand elements" have been blamed for several gun and ping pong bomb attacks near the PDRC former rally site at Ratchadamnoen Avenue, as well as for the violence that erupted at Ramkhamhaeng University and the clashes between police and demonstrators at the Thai-Japanese Stadium late last year.
National Farmers Council activist Khongjate Promnampon said he believed a surge in violence would be a sign that those in power were growing desperate.
He said they may want to terrorise demonstrators to force their opponents to the negotiating table.
If they fail, their ultimate aim is to provoke violence to invite a military putsch, Mr Khongjate said.
However, he believed the military is wise to the trap and would remain cautious.
Mr Abhisit yesterday said threats of violence are aimed at sapping the strength of the demonstrators, insisting they would never give in to such threats and intimidation.
NSPRT coordinator Uthai Yodmanee said violence will only undermine trust between the public and the government.
He said police were not doing enough to apprehend those who attacked the demonstrators.
There were no signs of trouble yesterday at the two protest targets named by Mr Uthai on Monday, the stock exchange and the central Bangkok offices of Aerothai, which is in charge of air traffic control communication for aircraft using Thai air space.