Thailand's king signed a new constitution in a ceremony on Thursday, an essential step towards holding an election that the military government has promised to restore democracy after a 2014 coup.

In a nationwide broadcast, King Maha Vajiralongkorn was seen signing the document, giving it royal endorsement and setting in motion a process for Thailand's next general election, expected late next year.

"The constitution of the Kingdom of Thailand is promulgated," an officer with the Royal Scribes Bureau said at the ceremony, on behalf of the King.

"May the Thai people be united in following and protecting the constitution to maintain democracy and their sovereignty."

The constitution is Thailand's 20th since the end of absolute monarchy in 1932 and critics of army rule say it will still give the generals a powerful say over Thai politics for years, if not decades. It replaces an interim constitution put in place after the 2014 coup.

Thais approved the long-awaited constitution at a referendum last August, but the palace requested changes in January after King Maha Vajiralongkorn took over from his revered late father, King Bhumibol Adulyadej, who ruled for more than seven decades.

One of the changes was to allow the king to travel abroad without appointing a regent. The king has spent much of the past few years in Germany, where he has a son in school.

Other changes, made by the junta and the military-appointed assembly at the request of the palace, have not yet been made public. Prime Minister Prayuth Chan-ocha has said the changes affect only the king and do not impinge on people's rights.

The constitution is now due to be published in the Royal Gazette, after which it becomes law.

There are still many steps before a general election can be held, or even until a ban is lifted on party politics. According to the timeline set out in the constitution, it could be late 2018 before a ballot and unforeseen delays are still possible.

The army initially promised an election in 2015, after seizing power in the name of ending political turmoil from a government run by Yingluck Shinawatra, sister of Thaksin Shinawatra, a populist leader ousted in 2006.

Thailand's main political division remains between a Bangkok-based strongly royalist and pro-army elite and poorer supporters of the Shinawatra's movement, particularly from the rural north and northeast.

One of the most controversial provisions of the new constitution is for the outgoing military government to appoint a senate that will have a say in appointing the prime minister.

The junta has argued the measure is necessary to prevent coups in a transition period after the election. Thailand has had 12 successful coups in the past 85 years.

Thursday is a public holiday in Thailand to mark the establishment of the Chakri dynasty 235 years ago. The current king, himself a former soldier, is also known as King Rama X in the dynasty.