Protesters gather ahead of pro-democracy rally in tense Bangkok

More than a thousand protesters gathered in Bangkok ahead of a pro-democracy rally on Wednesday, with tensions high in Thailand after the arrest of 21 pro-reform activists a day earlier.

The activists are part of a movement that has organised rallies since July, calling for an overhaul of the government and the resignation of Premier Prayut Chan-O-Cha. Some have also demanded reforms to the unassailable monarchy.

Anon Numpa -- a prominent activist and one of the key figures demanding royal reform -- urged the people gathering at the capital's Democracy Monument to remain calm and respectful, with the motorcade carrying the King and Queen scheduled to pass by in the afternoon.

"There will be provocations from the other side so please trust in me -- we don't want to clash with anyone," he said. "When the royal motorcade arrives, don't utter swear words."

The gathering comes a day after police clashed with protesters staging an impromptu rally at the same site, arresting 21 activists who were "uncooperative" as officials tried to empty the area for King Maha Vajiralongkorn's motorcade.

Dozens of remaining protesters raised a three-finger salute as the royal motorcade drove by on Tuesday, a gesture the pro-democracy movement has borrowed from the popular "Hunger Games" films.

By 10 am (0300 GMT) Wednesday, more than a thousand protesters had gathered there again, Bangkok police said. Some 14,000 police were deployed to control the situation.

'I have to do this'

Some pro-democracy protesters linked arms to form a chain and prevent authorities from stopping leaders setting up a stage earlier Wednesday.

"This could be the last fight for Thailand's democracy," said 18-year-old Attaporn, who travelled from the kingdom's northern Pichit province to join the rally.

"I have to do this if I want a better future."

The youth-led pro-democracy movement is the latest in Thailand's modern history, which has seen a cycle of opposing political factions taking to the streets in violent protests followed by coups from a putsch-happy army.

Wednesday's demonstration is intended to commemorate the 47th anniversary of the October 14, 1973 student uprising against a hated military dictatorship during which 77 people were killed.

The current protesters are demanding the removal of Prayut -- a former army chief who masterminded the 2014 coup -- a rewrite of a military-scripted constitution, and for the government to stop harassing political opponents.

On Wednesday, hundreds of counter-protesters wearing yellow -- the royal colour -- gathered near the Royal Thai Army headquarters, about a kilometre away from the Democracy Monument.

So far during the movement, more than two dozen protesters have been arrested and charged with sedition before being released on bail.