The leader of Uganda’s opposition, Kizza Besigye, has been detained for the fourth time in eight days, after he failed to unseat President Yoweri Museveni in an election observers say was marked by intimidation and a lack of transparency.

Ingrid Turinawe, a senior official with Besigye’s FDC party, said he was being prevented from gathering evidence as part of his challenge to the election result.

“They should leave him to be free because he has only 14 days to petition the court. He has to collect evidence,” she said.

President Yoweri Museveni. Photograph: Isaac Kasamani/AFP/Getty Images

Uganda’s electoral commission declared Museveni, who has been in power since 1986, the winner of the vote on 18 February, with 60.8% to Besigye’s 35.4%.

Besigye and other domestic critics said the result were rigged, while the European Union observer mission said the vote lacked transparency and had been conducted in an “intimidating” atmosphere.

Police said on Monday they believed Besigye was preparing to lead supporters to the commission to collect the official results and had not obtained proper consent.

“Today, [Besigye] had mobilised a group of youth to storm the electoral commission. We had information that they had planned to cause violence in the city,” said police spokesman Patrick Onyango.

The US secretary of state, John Kerry, has voiced concern over the harassment of opposition figures and the shutdown of social media in Uganda, where Facebook, Twitter and the WhatsApp messaging service experienced outages on election day.

Museveni, 71, has dismissed the idea that the commission had favoured him and his National Resistance Movement and has told foreign observers not to lecture him.

“I told those Europeans … I don’t need lectures from anybody,” Museveni told reporters outside his country home in Kiruhura, south-western Uganda.

He has presided over strong economic growth but is accused of clamping down on dissent and failing to tackle rampant corruption. Critics also say he wanted to rule the nation of 37 million people for life, emulating other African leaders who refuse to give up power.

Museveni last clashed with western donors in 2014, when Uganda passed a law that imposes harsh penalties on homosexuality.