BANGKOK (Reuters) - A Thai court jailed seven activists on Friday for campaigning against a military-backed draft constitution which will be put to a referendum in August.

Authorities arrested 13 people on Thursday while they were handing out leaflets urging people to vote against the charter. Six were released on bail, a lawyer for the group said, while the rest chose not to post bail.

"Six have been released but seven will be jailed," said Kumklao Songsomboon of the Thai Lawyers for Human Rights group.

In a rare show of unity, political parties on both sides of Thailand's divide have said the draft constitution is undemocratic and will enshrine military power and hobble civilian politics.

The junta has banned criticism of the charter in the run-up to the vote. It had already issued rules in May barring anyone from campaigning for either side ahead of the Aug. 7 referendum. Those who violate the rules face up to 10 years in prison.

The referendum will be the country's first return to the ballot box since junta chief Prayuth Chan-ocha took power in a 2014 coup which he said was needed to end months of street protests that had paralyzed the government.

Amnesty International in a statement on Friday urged Thailand to end its crackdown on activists.

"These crude tactics represent the latest in series of attempts by the Thai military authorities to muzzle dissent," said Champa Patel, Amnesty International's Senior Research Adviser for South East Asia and the Pacific.

"If a small group of activists cannot hand out leaflets, then what hope is there that the rights to freedoms of expression and assembly will be respected in the run up to the referendum?"

(Reporting by Amy Sawitta Lefevre and Panarat Thepgumpanat; Editing by Dominic Evans)