Thaksin, an influential political figure despite being ousted from power 18 years ago, reported himself to prosecutors just before 9 a.m. and the indictment process has been completed, Prayuth Bejraguna, a spokesperson for the Office of the Attorney General, said at a news conference.

A car believed to be carrying Thaksin arrived at the Criminal Court in Bangkok but he did not come out to meet reporters, and it is unclear whether he went to the court or the nearby prosecutors' office.

His lawyer Winyat Chatmontree told reporters that Thaksin was ready to enter the judicial process and that he has prepared a request for his release on bail.

The law on defaming the monarchy, an offense known as lese majeste, is punishable by three to 15 years in prison. It is among the harshest such laws globally and increasing has been used in Thailand to punish government critics.

Thaksin, now 74, was ousted by an army coup in 2006 that set off years of deep political polarisation. His opponents, who were generally staunch royalists, had accused him of corruption, abuse of power and disrespecting then-King Bhumibol Adulyadej, who died in 2016.

He was originally charged with lese majeste in 2016 for remarks he made a year earlier to journalists in South Korea. The case was not pursued at that time because he went into exile in 2008 to avoid punishment from other legal judgments he decried as political.

He voluntarily returned to Thailand last year and was immediately taken into custody for convictions related to corruption and abuse of power, but served virtually all of his sentence in a hospital rather than prison on medical grounds. He was granted release on parole in February.

Thaksin returned as the Pheu Thai party, seen as his political machine, joined hands with its longstanding rivals in the conservative establishment to form a government. The minimal punishment that he faced was interpreted as part of a deal to keep the progressive Move Forward party that finished first in last year's election out of power, though no deal was publicly acknowledged.

Thaksin has maintained a high profile and is seen as the unofficial power behind the Pheu Thai-led government. He has travelled the country making public appearances and political observations that could upset powerful figures on the establishment side.

Consequently, prosecution of the long-ago lese majeste case is seen by some analysts as a warning from his powerful enemies that he should tone down his political activities.

His case is just one of the several that have complicated Thai politics since the Pheu Thai government took office after the Senate, a conservative, military-appointed body, successfully blocked Move Forward from taking power last year.

Move Forward is now facing dissolution after the Election Commission asked the Constitutional Court to rule whether it is guilty of attempting to overthrow the system of constitutional monarchy by campaigning to amend the lese majeste law.

Prime Minister Srettha Thavisin, who is from Pheu Thai, meanwhile, is being probed over his appointment of a Cabinet member who had been imprisoned for bribery. If found culpable, Srettha could be forced out of his position.

Thailand's courts, especially the Constitutional Court, are considered bulwarks of the royalist establishment, which has used them and nominally independent state agencies such as the Election Commission to cripple political opponents.

The Constitutional Court on Tuesday is holding hearings on both Move Forward's and Srettha's cases. It will also rule whether the partially completed, three-stage voting process to select a new Senate is legal.

The term of the current Senate, appointed by the junta that toppled a previous Pheu Thai government in 2014, expired last month, opening up an opportunity to make its membership more democratic.

The votes could be annulled if the court finds the election process unconstitutional, which would allow the military-installed senators to remain on an interim basis until a new process can replace them.

Forty members of the interim Senate were behind the petition against Srettha, a move that is seen as favouring a pro-military political party in the coalition government.

The situation is a stark reminder of challenges Pheu Thai is facing from forming alliances with its old enemies, said Napon Jatusripitak, a political science researcher and visiting fellow at Singapore's ISEAS-Yusof Ishak Institute. He said it also reflects “a highly lopsided balance of power between elected and unelected forces in Thailand”. “Thai democracy is once again being held hostage by forces that are unaccountable to public interests,” he said.