By Amedee Mwarabu Kiboko

KINSHASA (Reuters) - A U.S. citizen working as an adviser to Congo's leading opposition presidential candidate has been freed six weeks after being arrested during a street protest and will return to the United States, the general prosecutor said on Wednesday.

Darryl Lewis, who was detained on April 24 along with three other members of Moise Katumbi's entourage in the southern city of Lubumbashi, has been handed over to the U.S. ambassador in the capital Kinshasa, Victor Mumba Mukomo told reporters.

Katumbi, the former governor of mineral-rich Katanga province in the south, was indicted last month on charges of hiring mercenaries as part of a plot against the state.

He denies the accusations, saying they are meant to derail his campaign to succeed President Joseph Kabila in the vast Central African country's elections scheduled for November.

Congolese authorities said Lewis was arrested because he lacked a permit to work in Congo. He will return to the United States but his case will remain open, Mukomo said.

Lewis's lawyer Azarias Ruberwa confirmed that Lewis would return home and said U.S. officials were looking into the case.

Political tensions are running high in Congo, which has vast reserves of precious minerals, ahead of the election.

Kabila is ineligible to stand after serving two elected terms but opponents accuse him of plotting to hold onto power by delaying the vote or even changing the constitution to remove the term limit, as several African leaders have recently done.

The government has said the vote is unlikely to occur on time because of logistical and budgetary problems.

In May, Congo's highest court ruled Kabila could remain in power until elections can be held.

Protests against any delay have already turned violent and authorities have arrested dozens of critics of Kabila, who took power when his father Laurent Kabila was assassinated in 2001.

Congo has never had a peaceful transfer of political power.

(Writing by Marine Pennetier; Editing by Makini Brice and Mark Heinrich)