A group of demonstrators wearing helmets and other protective gear reached the line of concrete barriers and razor wire placed near Bang Krabue intersection in front of the parliament in Dusit district at about 2:30pm. They then attempted to cross the barricade, throwing bags of paint and smoke bombs towards rows of riot police on the other side.
Police used a water truck to fire a salvo from water cannon in order to force them back.
After the brief confrontation ended, police tried to open negotiations with a representative of the approaching men, who were apparently acting as guards for the demonstrators.
Pol Col Kissana Phathanacharoen, deputy police spokesman said police had warned demonstrators not to destroy the barricades. When they did not back off, police fired four rounds from the water cannon.
The first two rounds consisted of plain water. When that failed to deter the demonstrators, he said, police then used the water cannon to fire a liquid that included tear gas solution.
Rally leaders earlier announced a plan to stay overnight at the parliament while waiting for MPs to accept for deliberation a proposed constitutional amendment drafted by civil group Internet Law Reform Dialogue (iLaw). It was one of seven charter change drafts submitted to the parliament, with other drafts sponsored by government and opposition parties.
This morning the parliament started its debate on a constitutional amendment. Lawmakers are expected to discuss the issue for two days before voting on whether to accept any of the proposed drafts for deliberation.