DAKAR (Reuters) - Seven Gambian opposition parties united late on Sunday behind businessman Adama Barrow, endorsing him as the main challenger to President Yahya Jammeh in elections scheduled for December.

Barrow, 51, became leader of the United Democratic Party (UDP), the country’s largest opposition party, in September. He succeeded Ousainou Darboe, jailed in July for three years following rallies calling for electoral reform.

“I am overwhelmed and will work with the opposition and the Gambian people to make the Gambia a better place for all of us,” Barrow said in an acceptance speech at a convention of seven opposition parties.

Jammeh has ruled the small West African state of 2 million people for 22 years. Human rights groups accuse him of systematically repressing dissent, straining relations with Western donor countries.

Nearly 50 protesters were jailed in April and May during the electoral reform rallies, and in August two UDP members died in prison, prompting calls for an investigation from the United Nations and the United States.

The government denied allegations that one of the pair was tortured.

In addition to Jammeh and Barrow two other candidates - independent Isatou Touray and Mama Kandeh, who has the support of an eighth opposition party - have said they will stand in the Dec. 1 vote.

The country’s electoral commission has asked candidates to submit their applications between Nov. 7-10.