By Amy Sawitta Lefevre and Panarat Thepgumpanat

BANGKOK (Reuters) - Thai student protesters will use social media tools, from Facebook to Japanese messaging app Line, to persuade voters to reject a military-backed draft constitution, some of the activists said on Monday.

The students rank among the most vocal critics of Thailand's military government ahead of an Aug. 7 referendum to approve a contentious draft constitution that critics fear could cement military control over civilian politics for years.

The activists told Reuters social media campaigns offer an edge over street action, such as a protest on Sunday in the capital by 43 civil and student groups hemmed in by government limits on public assembly and campaigning.

"We have a safe space that people can access, and that is the web," said Rangsiman Rome, 24, a member of the New Democracy Movement (NDM), who was detained briefly this month for handing out leaflets urging rejection of the charter.

More than half of a population of 67 million people uses the internet in Thailand, and many people have multiple mobile phones.

"One hundred percent of our public relations strategy lies with social media," added Rangsiman, a law student at Thammasat University in Bangkok.

His group has more than 71,000 followers on its Facebook page, which urges people to turn out for the vote, along with charts showing why they should reject the draft.

The military government says the proposed constitution would heal Thailand's caustic political divide and it is relying on a more traditional campaign to woo support, using broadcasts of patriotic songs and television programs.

It has reacted to criticism of its political plans by invoking a law that sets a 10-year jail term for campaigning ahead of the referendum. In the first such case in April, it targeted a group for Facebook posts criticizing the draft.

'NOT CONCERNED'

But the government is unperturbed by the social media campaign, said Suwaphan Tanyuvardhana, a minister in the Prime Minister's Office, adding that Thailand was determined to hold a general election next year no matter what.

"Whether the draft passes or not, Thailand will go to a general election," Suwaphan told Reuters.

"We're not concerned by what's happening on social media."

For more than a decade, Thailand has been divided between rival camps, one led by former populist premier Thaksin Shinawatra, who was toppled in a 2006 coup and later went into self-exile.

Against Thaksin is the royalist and military establishment, which accuses him of poisoning politics with nepotism and corruption, charges he denies.

Politicians on both sides have derided the draft as aimed at entrenching the power of the military.

For many Thais, the students said, the referendum is also a vote on the legitimacy of military rule since a May 2014 coup.

"The constitution is just one small point, but the bigger point is our system of governance," Rangsiman said.

The NDM wants the junta to resign if its draft is blocked, said another member, Pakorn Areekul, although Prime Minister Prayuth Chan-ocha has ruled out such a move.

The students will meet voters ahead of the referendum to emphasize the charter's drawbacks.

"We'll go and meet people face-to-face but we'll do this away from the media glare," said Pakorn, 27. "There are a great many people who remain undecided."

(Additional reporting by Patpicha Tanakasempipat; Writing by Amy Sawitta Lefevre; Editing by Clarence Fernandez)