By Cheikh Sadibou Mane

BANJUL (Reuters) - Gambian authorities cut the internet, barred international calls and sealed land borders for an election on Thursday that poses the first serious challenge to President Yahya Jammeh since he seized power in a coup 22 years ago.

Jammeh, who this week said only Allah can remove him from office, has pledged to rule the tiny riverside West African nation for "a billion years".

He has made frequent headlines by claiming to have a herbal cure for AIDS that only works on Thursdays, declaring Gambia an Islamic republic and threatening to slit the throats of homosexuals.

A Western diplomatic source said the National Intelligence Agency had informed them that internet and phone outages would last until Sunday.

The same source said that land borders had been shut, but sea and air remained open. Reuters could not independently verify the border crossing closures.

Dressed in flowing white robes, carrying strings of prayer beads and surrounded by soldiers, Jammeh voted in the capital Banjul without speaking to reporters.

Nearby, in the sleepy, palm-fringed streets, residents formed long lines outside polling stations where they cast ballots by dropping marbles into drums painted green, silver and purple for the three candidates, each with his picture on. Gambian officials say the system is designed to avoid spoiled ballots and to simplify the process for illiterate voters.

Britain, whose nationals swarm to the former colony's white-sand beaches in search of winter sun, advised its tourists on Thursday that international calls had been blocked in an apparent attempt to control scrutiny of elections.

Activist Jeggan Grey Johnson with Open Society Foundation called the outages a "deliberate attempt by the incumbent to control any sort of information sharing". Gambia's communications minister could not be reached for comment.

European Union observers have been barred from monitoring the polls; African Union observers have been admitted.

RARE OPPOSITION CHALLENGE

Rallies for the main opposition challenger, businessman Adama Barrow, have attracted crowds of thousands - a rare show of defiance towards a leader who rights groups say frequently imprisons and tortures opposition figures.

Barrow has promised to revive Gambia's economy, one of the region's most sluggish, end widespread human rights abuses and to step down after three years as a boost to democracy.

Jammeh's supporters deny allegations of atrocities and he frequently rails against Western interference in Africa's internal affairs.

The president has touted his record on healthcare and has won support among women owing to alleged herbal cures for infertility that he personally administers in the grounds of State House. He drew huge crowds during rallies across Gambia.

Gambia made headlines recently by withdrawing from the International Criminal Court, calling it the "Infamous Caucasian Court". Gambia also withdrew from the Commonwealth in 2013.

"My presidency and power are in the hands of Allah and only Allah can take it from me," Jammeh said earlier this week.

In April, small protests in Banjul calling for electoral reform led to dozens of arrests, including that of the leader of the main UDP opposition party, Ousainu Darboe.

Two UDP members have since died in custody; others remain in jail.

(Writing and additional reporting by Emma Farge and Edward McAllister in Dakar; Editing by Tim Cocks, Mark Trevelyan and Richard Lough)