BANGKOK - Thai protesters called on King Maha Vajiralongkorn to give up control of a royal fortune valued in the tens of billions of dollars on Wednesday as the latest in months of demonstrations focused squarely on the monarchy.

The protesters have broken a longstanding taboo by criticising the king and police summoned many of the best-known protest leaders on Tuesday on charges of insulting the monarchy, which can mean up to 15 years in prison.

"This law is ancient and barbaric. Every time it is used it damages the monarchy and the nation," said Parit "Penguin" Chiwarak, who is among those facing charges. He dressed in a yellow duck suit - echoing the giant rubber ducks that have become emblems of the protest.

The demonstration, joined by a couple of thousand people, had originally been scheduled at the Crown Property Bureau, which manages the royal assets.

But after police built siege barricades of shipping containers and razor wire, the venue was moved to the headquarters of the Siam Commercial Bank in which the king's stake of more than 23% makes him the biggest shareholder.

"The SCB shares should not belong to the king but the finance ministry, so the dividend can be used to develop the country," said Boss, 28, a protester who declined to give his full name.

Shares in the bank rose more than 2% on Wednesday, more than twice as much as the broader market.

The palace has made no comment since the protests began, but when the king was asked about the protesters recently he said they were "loved all the same".

Some of the king's critics quoted those words sarcastically after the summonses on charges of insulting the monarchy, which Prime Minister Prayuth Chan-ocha had said in June were not being used at the request of the king.

International human rights groups also condemned the use of the charges. Police sources said 15 protest leaders faced the charges, which they must acknowledge by the end of the month.

Responding to the criticism, government spokeswoman Rachada Dhnadirek said: "The government has been open-minded to rights and freedoms despite many imprudent expressions which offend the majority. The government must use its authorised powers."

Since July, protesters have been calling for the removal of Prayuth, a former junta leader. He rejects accusations that he engineered last year's election to keep hold of power he seized in a 2014 coup.

The protesters seek to make the king more accountable under the constitution as well as the reversal of changes which gave him personal control of some army units and the crown assets estimated to be worth more than $30 billion.

(Additional reporting by Chayut Setboonsarng Editing by Robert Birsel) ((matthew.tostevin@thomsonreuters.com; +66 2 648 9777; Reuters Messaging: matthew.tostevin.thomsonreuters.com@reuters.net))