An Australian senator on a fact-finding mission to Malaysia says he has been refused entry because authorities consider him a "security risk".
Nick Xenophon was detained at Kuala Lumpur airport on Saturday as he arrived for talks with Malaysian officials to discuss elections in June.
Immigration officials said the senator would be deported for taking part in a protest in Kuala Lumpur last year.
Mr Xenophon has been an outspoken critic of human rights in Malaysia.
A regular visitor to the country, he arrived in Kuala Lumpur as part of an Australian parliamentary fact-finding mission to assess whether forthcoming elections would be free and fair.
The delegation was scheduled to hold talks with several Malaysian parties, including opposition leader Anwar Ibrahim and officials from the election commission.
The independent senator from South Australia was taken into custody at Kuala Lumpur airport on Saturday morning and detained apparently under Malaysia's national security laws.
Immigration officials were polite but acting under orders "from above", Mr Xenophon said.
"I was told I am a security risk and I can't be allowed into the country. It's bizarre and extraordinary," the senator said.
"I've been here before [and] I've made statements about the state of Malaysian democracy previously. But on this occasion clearly someone high up in the Malaysian government doesn't want me here."
Mr Xenophon told the BBC he has repeatedly voiced fears that the country's forthcoming elections, due to be held in June, could be subject to "widespread election fraud and corruption".
"That clearly has been something that the Malaysian authorities don't like hearing," he said.
Last April, the senator was caught up in anti-government protests and reportedly tear-gassed by riot police at a demonstration in Kuala Lumpur demanding democratic reforms in April last year.
He also observed the trial of Mr Anwar, who was acquitted on sodomy charges.
The Malaysian government confirmed Mr Xenophon would be sent back to Melbourne over his alleged involvement in the 2012 protest.
"Malaysia is a free and democratic country, but no-one is above the law," immigration chief Alias Ahmad said in a statement.
"Authorities will take the appropriate action against any individual deemed to have violated national laws."
Australian Foreign Minister Bob Carr said his government was seeking Mr Xenophon's immediate release and had raised the issue with the Malaysian government.
"Senator Xenophon's detention is a surprising and disappointing act from a country with which Australia routinely maintains strong diplomatic relations," Mr Carr said.
"Our High Commissioner Miles Kupa has now made direct contact with Senator Xenophon at the airport and is seeking his release."
Other members of the Australian delegation have pulled out of the scheduled visit in response to the detention.