National Super Alliance (NASA) supporters demonstrate in Nairobi, capital of Kenya. Picture: Xinhua/John Okoyo

Nairobi - A Kenyan government watchdog said on Friday it was investigating whether police had assaulted students during protests this week at the University of Nairobi over the detention of an opposition lawmaker.

Police fired tear gas on Thursday at the protesting students. Video footage posted on social media later in the day showed uniformed officers outside dormitories and inside classrooms using batons to hit people who did not appear to be involved in the campus protests.

It was the latest crackdown by police on protests since an August 8 presidential election that was later annulled by the Supreme Court. A re-run of the vote has been set for October 27.

The Independent Police Oversight Authority (IPOA) "this morning noted from social media reports of an incident in which members of the National Police Service allegedly stormed the University of Nairobi and assaulted students at the institution," the authority said on its Twitter feed.

It requested that any member of the public come forward to provide information to aid the investigation.

The students had been protesting against the re-arrest of a lawmaker, Paul Ongili Owino, on Wednesday, shortly after he was released on bail on charges of subversion for calling President Uhuru Kenyatta a "son of a dog" at a campaign rally.

Kenya is a key Western ally in a region often roiled by violence. Preparations for the re-run of the election are being closely monitored for signs of instability, after at least 28 people were killed in unrest following the August 8 vote.

The IPOA watchdog was created in 2011, after police came under severe criticism for the number of protesters killed during demonstrations against disputed elections in 2007. The violence then killed around 12 000 people.

But local and international rights groups say the institution is struggling to fulfil its mandate to investigate allegations of police brutality in a country rife with reports from civilians of extrajudicial killings by security forces.

The IPOA has secured two convictions of police officers in the four years it has been operational.

The watchdog pledged last month to fast-track investigations into high-profile deaths such as that of a baby allegedly killed by police in the violence after the election.

But people familiar with the status of these investigations say the police are not cooperating with them, and that senior officials in the police force and the interior ministry insist officers killed only thieves and thugs.