The demonstration was organized by a group called Pitak Siam and is led by retired army Gen. Boonlert Kaewprasit. He told the demonstrators not to be lured into violence by police in riot gear, the Bangkok Post reported.
Boonlert also alleged the government of Prime Minister Yingluck Shinawatra had used regional officials to deter many thousands more people from attending the protest by setting up checkpoints and laying spike belts on highways to discourage traffic.
He called Yingluck Shinawatra a "puppet" for her brother, former Prime Minister Thaksin Shinawatra, now living in exile after a military coup ousted him in 2006.
As the crowds thinned, police said 37 people had received minor injuries and 132 others had been arrested, the Post said. Officers said they confiscated knives and slingshots whose apparent use was to disable security cameras around the protest site.
Sunday, parliament is scheduled to hold a confidence vote on Yingluck's government, which was elected last year.