Pol Maj Gen Jirasan said more than 300 passenger vans from different areas of Bangkok and neighbouring provinces began arriving at Victory Monument at 9.45am. The main rally is at Democracy Monument, which is nearby.
The van operators complain the ongoing anti-government protests, led by PDRC secretary-general Suthep Thaugsuban, have affected business and they are losing money.
Meanwhile, a survey by Suan Dusit Poll found that many young people are fed up with the continuing political conflict on Thailand.
The pollster questioned young people for their views on Thai politics from Monday to Wednesday, involving 1,337 respondents across the country.
The pollster said 61.51% saw Thai politics as disorderly and boring, 19.25% believed that it obstructed Thailand's development and caused unhappiness, 15.09% said all politicians were evil, greedy, devious and lazy and they were bad examples for children, and 4.15% said the political situation had disrupted their school classes.
On ways to resolve the political situation, 42.15% said the different sides should talk and listen to each other in a well mannered way and work together, 21.90% said bad politicians to should be severely punished, 19.83% wanted adults to think more about young people, 16.12% said adults should learn to accept defeat and to forgive.