* Thousands march to hand over demands
* Protesters install symbolic plaque near Grand Palace
* Police say plaque could be illegal (Adds people line up to take pictures with plaque in paragraph 12, protesters call for measures in paragraph 21)
By Patpicha Tanakasempipat and Matthew Tostevin
BANGKOK, Sept 20 (Reuters) - Openly challenging the monarchy of Thai King Maha Vajiralongkorn, thousands of protesters marched in Bangkok on Sunday to present demands that include a call for reforms to curb his powers.
Protesters have grown ever bolder during two months of demonstrations against Thailand's palace and military-dominated establishment, breaking a longstanding taboo on criticising the monarchy - which is illegal under lese majeste laws.
The Royal Palace was not immediately available for comment. The king, who spends much of his time in Europe, is not in Thailand now.
The marchers were blocked by hundreds of unarmed police manning crowd control barriers.
Protest leaders declared victory after handing police a letter detailing their demands. Phakphong Phongphetra, head of the Metropolitan Police Bureau, said the letter would be handed to police headquarters to decide how to proceed.
"Our greatest victory in the two days is showing that ordinary people like us can send a letter to royals," Parit "Penguin" Chiwarak, told the crowd before it dispersed.
At the biggest demonstration in years, tens of thousands of protesters on Saturday cheered calls for reform of the monarchy as well as for the removal of Prime Minister Prayuth Chan-ocha, a former junta leader, and a new constitution and elections.
Shortly after sunrise on Sunday, protesters cemented a plaque near the Grand Palace in Bangkok in the area known as Sanam Luang, or Royal Field.
It reads, "At this place the people have expressed their will: that this country belongs to the people and is not the property of the monarch as they have deceived us".
Government spokesman Anucha Burapachaisri said police would not use violence against protesters and it was up to the police to determine and prosecute any illegal speech.
Bangkok authorities would need to determine whether the plaque was illegal, and if it was it would be removed, Bangkok's deputy police chief Piya Tawichai told reporters.
After the protest, people queued up to take pictures next to the plaque, which also features a hand giving the three-finger salute adopted by pro-democracy protesters.
But far from all Thais support the new plaque, which resembles one that had commemorated the end of absolute monarchy in 1932 and which was removed from outside a royal palace in 2017, after Vajiralongkorn took the throne.
Prominent right-wing politician Warong Dechgitvigrom said the actions of the protesters were inappropriate and that the king was above politics.
"It didn't achieve anything," he told Reuters. "These actions are symbolically against the king, but the king is not an opponent."
Thai authorities have said criticising the monarchy is unacceptable in a country where the king is constitutionally "enthroned in a position of revered worship".
Protests that began on university campuses have drawn increasing numbers of older people. That includes "red shirt" followers of ousted populist Prime Minister Thaksin Shinawatra who had clashed for years with pro-establishment "yellow shirts" before Prayuth seized power in 2014.
"The new generation is achieving what their parents and grandparents didn't dare. I'm very proud of that," said Somporn Outsa, 50, a red shirt veteran. "We still respect the monarchy, but it should be under the constitution."
Protesters say the constitution gives the king too much power and that it was engineered to allow Prayuth to keep power after elections last year. He says that vote was fair.
The next protest is scheduled for Thursday. Protest leaders called on Thais to take Oct. 14 off work to show their support for change.
Other measures they sought for were for people to withdraw deposits from Siam Commercial Bank, in which the king's Crown Property Bureau owns more than 23% of the shares, and to stop standing for the royal anthem in cinemas.
(Additional reporting by Panarat Thepgumpanat; Editing by Chris Reese, William Mallard and Alex Richardson)
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