It was unclear if those hurt were anti-government protesters or security guards with the People's Democratic Reform Committee, which is organizing the rally, the Bangkok Post reported.
The protests come in advance of Feb. 2 elections ordered by caretaker Prime Minister Yingluck Shinawatra. The election commission has advised her to ask King Bhumibol Atlantic to stop the election over concerns of political deadlock and rising violence in advance of the election.
Shinawatra has dissolved parliament and proposed creation of a national reform council to end months of political gridlock. Opposition parties want reforms to take place before the elections, Voice of America reported.
The Monday protest, led by former Deputy Prime Minister Suthep Thaugsuban, aims to block major highways and prevent government offices from functioning.