DUBAI (Reuters) - Leading Iranian opposition figure Mehdi Karoubi, under house arrest since 2011, was hospitalized briefly after showing symptoms including weight loss, dizziness and nausea, his website said on Tuesday.

Karoubi, 75, and fellow reformist Mirhossein Mousavi ran for election in June 2009 and became figureheads for Iranians who protested against the vote they believed was rigged to bring back President Mahmoud Ahmadinejad.

The government denied wrongdoing and said foreign enemies had plotted to overthrow Iran's leaders by stirring up the protests, the biggest opposition demonstrations since the 1979 revolution.

Hardliners have asked the judiciary to execute the two opposition leaders, calling them "seditionists" who aimed to topple the clerical establishment. But authorities have chosen so far to isolate rather than officially arrest them.

"Mr. Karoubi was hospitalized for some hours in one of the hospitals belonging to the security institutions," his website Sahamnews said. "This action by the security institutions followed warning signs relating to Mr. Karoubi's physical condition."

Karoubi, Mousavi, and Mousavi's wife Zahra Rahnavard were confined to their homes in February 2011 after the opposition leaders called supporters of Iran's reformist "Green Movement" to rally in solidarity with uprisings in Arab countries.

None have been seen in public since.

Karoubi, a turban-wearing cleric and former parliament speaker, was transferred to another location later in 2011. His family said in a statement published by Sahamnews on Tuesday that they were worried for the elderly cleric's health.

"We expect that his right to access independent treatment will be officially recognized, so that medical follow-up under the supervision of trusted family doctors can take place in one of Tehran's hospitals," the statement said.

Karoubi suffered from respiratory complications in November 2011, Sahamnews reported at the time.

Mousavi, 70, Iran's prime minister in the 1980s, was hospitalized briefly in August 2012 to treat a heart problem.

(Reporting by Yeganeh Torbati; Editing by Robin Pomeroy)