BANGKOK (REUTERS, AFP) - Thousands of people protested in Bangkok on Saturday (Nov 14) in the latest in months of anti-government demonstrations that have also called for reforms to Thailand's powerful monarchy.

Some of them later scaled a Bangkok monument to unfurl a giant banner scribbled with anti-government slogans and calls to reform the monarchy.

A few kilometres away, thousands of royalists gathered in yellow shirts and waved Thai flags as they waited to greet King Maha Vajiralongkorn, who arrived in a motorcade with Queen Suthida for the opening of a new railway line.

Protesters are reported to have turned their backs on the motorcade as it passed through central Bangkok and gave the three-fingered "Hunger Games" salute of pro-democracy campaigners.

The King was greeted with a show of support when he arrived at the rail ceremony in the west of the city, where people in yellow shirts waved national flags and chanted "long live the King".

"He told me to show children how important the unity of the country is," said Donnapha Kladbupha, 48, a teacher who posed for selfies with the King.

The initial focus of protests that began in July was to seek the removal of Prime Minister Prayut Chan-o-cha, a former junta leader.

"Not only is he incompetent, he also lacks legitimacy," activist Sombat Boonngamanong said from loudspeakers on the back of a truck wearing a pirate hat. "Thailand has not progressed because of Prayut."

ome 2,500 protesters gathered at Democracy Monument in Bangkok, according to police, putting on songs and dances mocking the government.

Mr Prayut's government holds the majority in Parliament because his junta picked the entire upper house before an election last year that opponents say was designed to keep him in power. He says the vote was fair.

Police said they would not use violence to crack down on demonstrators and deployed 5,100 troops to maintain order.

But last week, thousands were met with water cannons when they marched to the Grand Palace to demand curbs to the monarchy's power.

Two kilometres away, thousands of royalists were waiting for the King's arrival where he was due to attend the opening ceremony of a subway station.

The protesters turned their backs and raised their hands in three-finger salutes - a pop culture reference to the "Hunger Games" movies - as the royal motorcade drove past.

Demonstrators have increasingly called for reforms to the powerful monarchy, breaking a longstanding taboo against criticising the institution.

"Some people want to bring him down, but we have come out to support him and show that all Thai people love him," said Donnapha Kladbupha, 48.

The Royal Palace has not commented since the start of the protests, but the King said two weeks ago that the protesters were still loved and that Thailand was a land of compromise.

Criticism of the monarchy can be punished with 15 years in jail under Thailand's lese majeste laws, but it has become widespread in recent weeks.