Seven people were hurt when unidentified gunmen on motorbikes opened fire on anti-government demonstrators during protests in Bangkok.
One of the injured remains in a critical condition after two separate night-time attacks on the main protest site in the capital on Saturday and Sunday.
Police said a gunman also fired shots at the opposition party headquarters in a separate incident, although no casualties were reported.
Protesters launched ‘Black Monday’ shutdown today with theThai army and police fearing further violence.
Seven major intersections have been blocked by the anti-government protest movement, which has erected stages and piles of sandbags across the roads, reports the BBC.
Protesters say they will remain in place for several days.
The Foreign Office is advising visitors that the "situation is unpredictable, and further protests are expected".
"Political demonstrations continue in and around Bangkok and elsewhere in Thailand. Some of these have been violent, including the use of firearms, and there have been casualties," it said.
It said the increased protests on Monday may cause major disruption to travel on main roads in and around Bangkok.
"You should take extra care and avoid all protests, political gatherings and demonstrations. If you’re travelling to the airport, allow extra time to take account of possible transport delays, and consider using the airport rail link. Monitor local news and social media for developments."
The Tourism Authority of Thailand has said the "utmost efforts" will be taken to ensure tourists are not affected by the shutdown.
Special lanes have been set up for tourist buses and ‘Tourist Friend Centres’ (dial 1672 locally) have been established at the main BTS & MRT stations to assist overseas visitors.
Protesters have promised they will not disrupt the tourist infrastructure, including the two airports, Suvarnabhumi and Don Mueang.
But air travellers have been advised to allow extra time for check-in and getting to the airport.
