HANOI (Reuters) - Vietnam's President Tran Dai Quang, a former chief of internal security who became one of the most high-profile leaders in the communist-ruled country, died on Friday after an illness, state media said.

Quang's rise, at a time of growing dissent on social media and challenges to the party's authority, signalled limits to Vietnam's liberal agenda, despite a growing openness to social change and sweeping economic reform, experts say.

Quang, 61, died in a military hospital in Hanoi of a "serious illness despite efforts by domestic and international doctors and professors," Vietnam Television said.

He had hosted a reception for China's Supreme Court chief on Wednesday, state-owned newspaper Vietnam News said.

Quang had been ill for months, said former health minister Nguyen Quoc Trieu, who now oversees healthcare for high-ranking officials.

"He began showing symptoms of illness in June last year and has been treated in Japan six times since," Trieu told Reuters.

"He suffered from a kind of highly virulent virus, for which there has not been any efficient treatment."

Vietnam has no paramount ruler and is officially led by four "pillars": its president, prime minister, the chief of its Communist Party and the national assembly chair.

The president has a more ceremonial role, although he sits on the powerful decision-making politburo.

Before his April 2016 appointment as president, Quang had been minister of public security, heading an organisation with broad powers responsible for intelligence gathering and thwarting domestic and foreign threats to the party.

ILLNESS RUMOURS

Posts about the death of Quang, once tipped by some analysts as a potential general secretary, spread quickly on Facebook before state media took the rare step of announcing it within two hours of the event.

Rumours of his illness had circulated on social media for months, and there was no significant reaction in Vietnam's benchmark index .

At one of Quang's last appearances, during a visit to Hanoi by Indonesian President Joko Widodo on September 11, the president appeared visibly unwell and stumbled as he stepped on to a platform to inspect a guard of honour.

Quang hailed from a small farming community 115 km (70 miles) south of Hanoi, rising through party ranks to become a police general and Politburo member.

"We are saddened to hear the news that the president has died," said Bui Duc Phi, council chairman of the village where he was born.

Vietnam's constitution provides for the vice president to perform the president's duties should he be unable to work.

"Vice president Dang Thi Ngoc Thinh will handle his duties until the National Assembly elects a new president," said prominent lawyer Tran Vu Hai.

The national assembly is set to meet in late October, but the date could be brought forward, he added.

(Additional reporting by Kham Nguyen, Mai Nguyen and Minh Nguyen; Writing by Raju Gopalakrishnan; Editing by Neil Fullick and Clarence Fernandez)