Thousands of demonstrators occupied Ratchaprasong intersection in Bangkok on Wednesday demanding reform of the monarchy and the release of detained leaders.

Earlier, the  United Front of Thammasat and Demonstration posted a Facebook message urging a peaceful rally, days after one of the most violent demonstrations since they began last year.

More than 30 civilians and police were injured in a clash at a protest near the palace on Saturday, after police used water cannon, tear gas and rubber bullets to disperse the rally.

The protesters have also demanded curbs to the power of His Majesty the King and the end of a lese majeste law that gives up to 15 years in prison for insulting the monarchy.

Protesters on Wednesday peacefully occupied Ratchaprasong, one of Bangkok's busiest street intersections, and cheered as some leaders demanded monarchy reforms and called for their "friends" to be freed from jail.

"This is a long and tiring fight. We must fight together along this path to democracy," Attapon Buapat, who was indicted for lese majeste earlier this month but later released on bail, told the crowd.

"Change has begun. They could only slow it down but never stop it."

Criminal cases have been mounting against protest leaders. The most prominent ones have already been jailed pending trial for insulting the royalty.

Thailand's youth protest movement emerged last year and has posed the biggest challenge to the government of Prime Minister Prayut Chan-o-cha, the former army chief who seized power in 2014 from an elected government.