He was released on the two previous occasions without being charged.

His latest arrest came as he was about to hold a press conference.

The US embassy in Uganda condemned the crackdown in a post on its Twitter account.

After Mr Besigye was briefly detained on Thursday, the US State Departmentsaid the arrest called into question "Uganda's commitment to a transparent and free election process".

It also criticised a temporary shutdown of social media, chat apps and mobile money services.

President Museveni said the decision had been taken for security reasons, and to prevent people from "telling lies".

Police raided the opposition headquarters ahead of a planned press conference

President Museveni currently leads with 62% of the vote, while Mr Beisgye has 33%, according to provisional results released by the electoral commission.

Voting is still taking place in a handful of places where the late arrival of election materials meant that polling stations did not open on Thursday.

Commonwealth election observer mission head, Nigeria's former President Olusegun Obasanjo, condemned the failure of voting stations to open on time.

"Delays of three, four, five and even six hours, especially in Kampala, are absolutely inexcusable and will not inspire trust and confidence in the system and the process," he said.

A candidate needs to secure more than 50% of the vote to avoid a run-off with the second-ranked contender.

Ugandans also voted in parliamentary and local elections.

Final results are expected by Saturday.

Voting at some polling stations was extended into a second day after delays

"Delays of three, four, five and even six hours, especially in Kampala, are absolutely inexcusable and will not inspire trust and confidence in the system and the process," he said.

A candidate needs to secure more than 50% of the vote to avoid a run-off with the second-ranked contender.

Ugandans also voted in parliamentary and local elections.

Final results are expected by Saturday.

Major presidential contenders:

• Kizza Besigye, 59, veteran opposition leader and once personal doctor to incumbent President Museveni. He has lost the last three elections
• Amama Mbabazi, 67, former ally of President Museveni and once prime minister and also served as defence, security and justice ministers
• Yoweri Museveni, 71, in power since winning a five-year guerrilla war in 1986 and he is one of Africa's longest-serving leaders. His final term was meant to end in 2006, but in 2005 he won a campaign to lift the constitutional term limits.