At least four local journalists were reportedly beaten during the incident outside the U.N. rights office in the capital, Kampala. Witnesses said two journalists suffered head injuries when military police attacked them with batons. 

The journalists were there to cover the filing of a complaint by Bobi Wine, who was runner-up in last month's presidential election. In his complaint, Wine cited allegations of rights abuses including abductions that took place during and after the election. 

Ugandan military spokeswoman Brig. Flavia Byekwaso, said on Twitter that the army “regrets that some journalists were injured as security carried out its duties.” 

Wine, a singer and lawmaker whose real name is Kyagulanyi Ssentamu, has challenged the outcome of the Jan. 14 vote that official results showed was won by incumbent President Yoweri Museveni. 

“The U.N. calls on the Government to immediately investigate this incident and ensure that those responsible are brought to justice,” the U.N. statement said, noting that the behavior of Ugandan forces contravenes an agreement with authorities regarding the safety of U.N. premises, staff and guests. 

The U.N. statement said that its office would “immediately study" Wine's allegations and would “take the appropriate actions.” 

Museveni, who has ruled Uganda since 1986, is often credited with presiding over relative peace and security throughout his tenure. But last month's election saw the most violence in recent years, with security forces accused of cracking down on Wine's supporters. Some critics now charge that Museveni's government is behaving like those of previous dictators, including Idi Amin. 

Wine on Monday released a list of 243 people he said were were abducted by state agents. Police have accused Wine's supporters of planning riots aiming to topple Museveni and have arrested Wine multiple times. 

The U.S. and the European Union have expressed concerns over Uganda's election. 

U.S. Ambassador Natalie E. Brown recently referred to “deep and continuing concern about the extrajudicial detention of opposition political party members, the reported disappearance of several opposition supporters, and continued restrictions” imposed on Wine's party.