BANGKOK, Feb 15 (Reuters) - Only three days before he was shot dead in his home on the Thai border, a top Myanmar rebel leader predicted heightened tension with the ruling military junta in the run-up to a May constitutional referendum.

In an interview with Reuters on Monday, Karen National Union (KNU) secretary general Mahn Sha Lar Phan said the junta's plan to push through its army-drafted charter would meet opposition not just from the former Burma's many rebel groups.

"They will face many difficulties with this referendum, because people know this referendum will make them slaves," the soft-spoken 64-year-old history graduate said, sitting in the same wooden seat where he would be shot dead three days later.

Dissident groups are already campaigning for a "no" vote, saying the as yet unfinished charter is an attempt by the junta to legitimise its grip on power after 46 years of military rule.

Even though Mahn Sha's assassination could be the result of an internecine vendetta, the predominantly Christian KNU leadership was quick to accuse Yangon's military regime of orchestrating the hit via a loyal Buddhist Karen splinter group.

"This is the work of the DKBA and the Burmese soldiers," his son Hse Hse said, referring to the Democratic Karen Buddhist Army.

According to Mahn Sha's neighbour, the two gunmen spoke Karen.

The Karen, an ethnic minority of about 7 million people, have been fighting for independence since 1949, one of the world's longest-running guerrilla conflicts. They are one of only three rebel militias not to have signed a ceasefire with the junta.

There had been no contact between the KNU leadership, based on the Thai border, and Myanmar's generals for a year, Mahn Sha said, adding that there appeared to be little immediate prospect of peace. His death makes the chances even more remote.

"If the regime declared a nationwide ceasefire, there would be no need to fight," he said. "But now, we need to protect our people and to protect ourselves."

Having graduated from Rangoon University in 1966 with a degree in history, Mahn Sha threw himself into the Karen liberation movement, rising to became its secretary-general in 2000.

"His determination for freedom and democracy lives on within us and within the Karen people," his four children said in a statement issued by the Burma Campaign UK, an activist group.

"He did not live to see freedom for our people, but his dream will be fulfilled. The Karen, and all the people of Burma, will be free".

Myanmar has been under military rule of one form or another since 1962, during which time it has been riven by dozens of ethnic guerrilla wars, funded in large part by revenues from opium sales from the notorious "Golden Triangle". (Editing by Darren Schuettler and Alex Richardson)