BANGKOK: An alliance led by Thailand's progressive Move Forward party on Monday signed an ambitious agreement to draft a new constitution, end monopolies and allow same-sex marriage, among other aims, but made no mention of a divisive royal insult law.

The opposition Move Forward and Pheu Thai parties dominated last week's election in a resounding defeat of conservative parties backed by a royalist military that has controlled government since a 2014 coup.

They are seeking to form a coalition government with six other parties, all of which signed the agreement on their objectives.

Move Forward's leader Pita Limjaroenrat said the pact was "about shared values and commonalities and shared agenda and accountability".

"All parties can propose their own policies but must not violate this agreement through ministries," he added in remarks made at a press conference after a day of negotiations.

The signing took place on the ninth anniversary of a military coup against a democratically elected government, which began a phase of military rule that the winners of the May 14 poll hope to end.

Move Forward was the surprise election winner, emerging with the most parliament seats with the help of young voters excited by an agenda that puts the party at odds with some conservative big business interests and institutions, including a plan to amend a lese-majeste law that punishes perceived insults of the monarchy with long jail sentences.

Other alliance members have expressed reservations, and Monday's agreement did not include a proposal to reform that law. But it affirmed the country's "status as a democracy under a constitutional monarchy framework, and the inviolable status of the monarch".

Pita said on Monday he did not think his party's independent attempt to push reform of lese majeste laws will put off the upper house, whose backing the coalition needs to appoint a prime minister and form a government.

"We have a team to explain how to amend it so it cannot be used as a political tool ... this will ease the concern of senators," he told reporters.

Separately, a royalist activist on Monday filed a complaint against Move Forward at the Election Commission over its plans to amend the law, arguing it would damage the monarchy.

The alliance's agreement includes most of Move Forward's flagship polices, such as a push for decentralisation of power and budget and to "cancel monopolies and support fair competition in trade in all industries".

It also takes aim at the military, calling for ending mandatory conscription and reform of the armed forces as well as the justice system and civil service.

Thailand's military has staged 13 coups since the end of absolute monarchy in 1932 and confrontation between the generals and civilian politicians and activists has been at the heart of recurring instability over the years.

The alliance will also seek reform of welfare and education, a balanced foreign policy and pursuit of laws to both control and endorse the use of cannabis, which Thailand legalised last year, despite confusion over regulations.

Pita is seeking to be prime minister at the head of the coalition but faces a challenge in winning the required support from more than half the combined lower and upper houses, which includes rivals with whom his party has clashed.

His alliance comprises 313 seats, but it needs backing from 376 legislators to vote Pita in. He will likely need to win over some of the 250 members of the conservative-leaning Senate, which was appointed by a junta and has often sided with army-backed parties.