Police have arrested Uganda's main opposition presidential candidate, a day after tightly contested elections, a BBC reporter at the scene says.

Kizza Besigye was detained during a raid on his party's headquarters in the capital, Kampala, as he was about to hold a press conference.

Tear gas was also fired outside the building, our reporter says.

With official results for nearly half of the polling stations announced, President Yoweri Museveni is leading.

He is running for a fifth term, after 30 years in power.

This is the third time in the space of a week that Mr Besigye has been detained.

The police have not said why he was arrested or where he is being held.

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 Department said 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 had justified the moves on security grounds and to prevent people from "telling lies".

The security forces denied journalists access to the offices during the raid

There is a heavy deployment of police and soldiers in neighbourhoods around the headquarters of Mr Besigye's Forum for Democratic Change (FDC) party, our reporter adds.

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.

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

Commonwealth election observer mission head 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.