DAKAR (Reuters) - The top U.N. regional official for West Africa on Thursday called for the release of dozens of political prisoners in Gambia as long-ruling President Yahya Jammeh submitted his candidacy for a fifth mandate ahead of a December election.

During a visit to the country on Thursday, Mohamed Ibn Chambas urged transparency and a "level playing field" ahead of polls which regional observers have previously boycotted, citing intimidation of the opposition.

"I entreat the government to release the thirty protesters involved in the April demonstrations, as well as the fourteen others who are in pre-trial detention ...," he said, referring to protests in favor of free speech and electoral reform.

He also sought probes into the deaths of two opposition leaders detained around the same period. Jammeh's critics say the deaths are suspicious but the government denies any wrongdoing.

Despite a rare show of unity from Gambia's opposition who have rallied behind businessman Adama Barrow, most observers still expect Jammeh to extend his 22-year rule on Dec. 1.

That is despite growing signs of discontent within the country such as the April protests and a rising number of departures from Gambia, which has the highest number of migrants arriving in Italy this year relative to its population.

Many others have drowned on the way.

Chambas added that it was "regrettable" that a meeting scheduled for earlier this week between Jammeh and the opposition that was intended to provide guidelines for fair elections had been canceled.

At the last election in 2011, regional bloc ECOWAS refused to send observers, citing intimidation of the opposition and the electorate.

However, Jammeh said on Thursday all observers were welcome.

"I urged the chairman of the Independent Electoral Commission (IEC), Alieu Momar Njai, to invite all those who doubt the credibility of the electoral process to come observe the whole election procedure," he said shortly after filing his candidacy to the IEC.

EU sources had previously said they had positive indications that a small team of election experts would be welcomed to the country.

That is despite signs of the former British colony's growing disenchantment with the West, such as its government's decision to withdraw from the International Criminal Court in October.

(Reporting by Emma Farge; Editing by James Dalgleish)