A Police Service of Northern Ireland (PSNI) spokeswoman said vehicles had been torched and explosive devices hurled at officers as disorder continued in the city centre. Four men have been arrested so far.

"A pipe bomb-type device was thrown at officers in the area of Free Derry Corner some time before 7:00pm (1800 GMT)," the spokeswoman said.

"It exploded. However, there were no injuries or damage caused to property."

A car was set alight, while a van was hijacked, as was another car. "A woman and her daughter were pulled from the vehicle," the spokeswoman said.

"Several petrol bombs were thrown at the Memorial Hall and at police Land Rovers," she added.

The unrest was ongoing in certain central streets.

"Attempts are being made to hijack vehicles in these areas," the spokeswoman said.

The unrest follows mainland Britain's worst rioting in decades, which started last Saturday in north London but soon spread to other parts of the capital and to cities beyond.

However, disturbances during the annual Protestant marching season are a regular occurrence in Northern Ireland.

As many as 15,000 people and 140 bands were set to turn out for the annual Apprentice Boys parade.

The BBC reported that the hijacked van was a Royal Mail postal van. Four masked men hurled petrol bombs inside the vehicle before running away, the broadcaster said.

Saturday's incidents followed a night of unrest in the city, in northwestern Northern Ireland on the border with the Republic of Ireland.

A 24-year-old man has been charged with riotous behaviour, possession of a petrol bomb and assault, while two men aged 18 and 19 have been charged with disorderly behaviour, as has a 32-year-old man who was also charged with resisting arrest, the PSNI said.

They are all due to appear at Derry Magistrates Court on September 9.

Copyright AFP (Agence France-Presse), 2011