* Turnout is strongest showing by Islamists in two decades
* Islamist leader calls for democracy
* Three days since government announced, protests dry up
* U.N. chief calls for "timely and credible" elections
(Adds Ban Ki-moon remarks, paragraph 15)
By Lin Noueihed and Tom Perry
TUNIS, Jan 30 (Reuters) - Thousands of Tunisians turned out on Sunday to welcome home an Islamist leader whose return from 22 years of exile indicated that his party would emerge as a major force in Tunisia after the ousting of its president.
The reception for Sheikh Rachid Ghannouchi, leader of the Ennahda party, at Tunis airport was the biggest showing by the Islamists in two decades, during which thousands of them were jailed or exiled by president Zine al-Abidine Ben Ali.
Ghannouchi was exiled in 1989 by Ben Ali, who was toppled on Jan. 14 by popular protests that have sent tremors through an Arab world where similarly autocratic leaders have long sought to suppress Islamist groups.
Protesters in Egypt demanding an end to President Hosni Mubarak's 30-year rule have been inspired by the example of Tunisia. Egypt's main opposition group is also Islamist, but played no part in organising the protests there.
Ennahda is expected to contest future legislative but not presidential elections, dates for which have yet to be set.
The Islamists were Tunisia's strongest opposition force at the time Ben Ali cracked down on them in 1989 but are thought not to have played a leading role in the popular revolt.
But at Tunis airport on Sunday, they were out in force.
Up to 10,000 young men and veiled women packed the arrival hall and car park. Some climbed trees and electricity pylons to catch a glimpse of the 69-year-old Ghannouchi, who says he has no ambition to run for state office.
"Oh great people who called for this blessed revolution, continue your revolution, preserve it and translate it into democracy, justice and equality," Ghannouchi told the crowd, to chants of "Allahu Akbar".
Ennahda supporters embraced each other in joy. A group of men performed prayers on a grass verge, a scene unthinkable in Tunisia just a few weeks ago.
Ennahda likens its ideology to that of Turkey's ruling AK Party, saying it is committed to democracy. Experts on political Islam say its ideas are some of the most moderate among Islamist groups.
Tunisia has imposed a secular order since independence from France in 1956. Habib Bourguiba, the independence leader and long-time president, considered Islam a threat to the state. Ben Ali eased restrictions on the Islamists when he seized power in 1987, before cracking down on them two years later.
The protests which dislodged Ben Ali and electrified the Arab world have largely dried up in the last few days following the announcement on Thursday of a new interim government purged of most of the remnants of Ben Ali's regime.
The security forces have tried to restore order to the capital, where confrontations between shopkeepers and protesters have indicated dwindling support for demonstrators on the part of Tunisians who want life to return to normal.
U.N. Secretary General Ban Ki-moon, speaking at the African Union summit in Addis Ababa, said the United Nations would be "pleased to help the people of Tunisia freely choose their leaders through timely and credible elections".
Ghannouchi told the crowd the path to democracy was "still long". "Unite and consolidate, democracy cannot happen without national consensus and development can only happen with justice and democracy," he said.
Ennahda activists wearing white baseball caps tried to marshal the crowds. Asked how they had managed to organise so quickly, one activist said: "Our activities were stopped, but you can't disperse an ideology."
Some Ennahda activists were among the political prisoners released under an amnesty granted by the interim government.
A handful of secularists turned up at the airport to demonstrate against the party, holding up a placard reading: "No Islamism, no theocracy, no Sharia and no stupidity!"
Ennahda and its supporters say they do not seek an Islamic state and want only the right to participate in politics.
"We want a democratic state," said Mohammed Habasi, an Ennahda supporter who said he had been jailed four times since 1991 for "belonging to a banned group".
"We suffered the most from a lack of democracy," he said.
Abdel Bassat al-Riyaahi, another Ennahda activist who returned from exile, said: "We were banned for 21 years ... but we came back with our heads held high.
"Thank God for the great Tunisian people." (Additional reporting by Hamuda Hassan, Abdelaziz Boumzar, Musab Kheirallah in Tunis and Aaron Maasho in Addis Ababa; Writing by Tom Perry; editing by Tim Pearce)