UN Secretary-General Antonio Guterres on Friday urged all sides in Senegal to avoid any further escalation of violence after fresh clashes between police and supporters of detained opposition leader Ousmane Sonko.

"The Secretary‑General is very much concerned about the recent violence that we've seen in Dakar and other parts of Senegal," his spokesman Stephane Dujarric told reporters.

"His call is very clear on all concerned in Senegal to avoid further escalation of the situation," he said. 

"The protests must remain peaceful, and the security and police forces must at all times operate... in line with international human rights standards," Dujarric added.

"Faced with peaceful demonstrators, security forces, police forces must allow those demonstrators to express their opinion and to express their will."

He was speaking after Senegalese police fired tear gas in clashes with Sonko supporters after a court-ordered him held in custody following two days of violence over his arrest.

At least one person has been killed in protests, shops attacked and cars torched on the streets of Dakar since Sonko was first arrested on Wednesday.