The United Nations said on Thursday it was "deeply concerned" by violence during protests in Kenya which left at least 16 people dead and others with injuries, calling for calm and restraint.

Wednesday's rallies began peacefully but descended into chaos as young men held running battles with police, lit fires and ripped up pavements to use as projectiles.

The marches were called to mark the one-year anniversary of anti-tax demonstrations which peaked when a huge crowd stormed parliament, and dozens were killed by security forces.

"We are deeply concerned by reports of several deaths of protesters and many more injuries – of protesters and police officers – during demonstrations in Kenya on Wednesday," UN human rights office spokesperson Elizabeth Throssell said in a statement.

Investigations

"We are concerned by reports that some protesters had gunshot wounds.

"Under international human rights law, lethal force by law enforcement officers, such as firearms, should only be used when strictly necessary."

At least 16 people were killed and thousands of businesses ruined.

The UN rights office noted that Kenya's Independent Policing Oversight Authority has announced it will launch investigations.

Rights of peaceful assembly

"We underscore the importance of prompt, thorough, independent and transparent investigations to bring those responsible to justice... We also reiterate calls for calm and restraint," said Throssell.

"The rights of peaceful assembly and freedom of expression are cornerstones of the rule of law and must be respected and upheld by all."