Ugandan military and police officers beat and seriously injured journalists on Wednesday as they covered the delivery of a petition about human rights violations to a United Nations (UN) office, a rights group said.

The local office of the UN said its Office of the High Commissioner of Human Rights had received the petition from opposition leader and pop star Bobi Wine and would immediately study the allegations and take appropriate action.

The security personnel assaulted the journalists as they covered Wine.

"This behaviour contravenes the agreement with the government ... regarding the safety and security of UN premises, personnel and guests," the UN office said in a statement.

Police said security personnel had dispersed "uncontrolled crowds" that had formed outside the UN office as Wine delivered the petition.

"Unfortunately, a few people sustained injuries including journalists. We have opened an inquiry ... to establish the circumstances under which the victims were injured," a police statement said.

At least 20 journalists were hurt in the attack, with at least four sustaining deep cuts on the head, Stephen Bwire, Secretary General of Uganda Journalists Union, told Reuters.

"We condemn this brutal, barbaric and heinous act," Bwire said. "The journalists were doing their duty and they were clearly identifiable as journalists, they were not hostile, they were politely covering the events."

Wine, whose real name is Robert Kyagulanyi, lost last month's presidential election to incumbent Yoweri Museveni, who has ruled Uganda since 1986.

Wine rejected the results and has filed a court case seeking annulment of the poll, which he says was rigged.

The 39-year-old has a large following among young people who are drawn by his youthful energy and the music that he sometimes uses to criticise government.

To try to stem his growing popularity, authorities have cracked down on his supporters, using detentions and beatings.

"Today we went to the @UNHumanRights office in Kampala to deliver a petition against the continuing abductions, torture and murder of our supporters," Wine tweeted on Wednesday. He accompanied the tweet with two images of some of the journalists who were assaulted.

Journalists have increasingly been targeted by security personnel. Bwire said they had documented at least 50 journalists injured by security personnel during the election period, including one who was killed.