Dozens of petrol bombs have been thrown at police during a night of disturbances in Londonderry.
Two stolen cars, one of which was hijacked, were also burnt out at "free Derry Corner" in the Bogside area.
The trouble came hours before the main Apprentice Boys parade through the city centre. The parade, however, passed off peacefully.
About 10,000 members and 130 bands took part in the demonstration in Derry on Saturday.
There was a highly visible police presence on the ground but no serious incidents were reported.
"The fact that petrol bombs appeared on the scene in such a short time - obviously someone had taken time to prepare for that eventuality"
During the disturbances in the city, about 50 petrol bombs were thrown, said the PSNI.
The trouble took place late on Friday night and in the early hours of Saturday morning.
Superintendent Dave Hanna said it appeared to have been planned.
"The fact that petrol bombs appeared on the scene in such a short time - obviously someone had taken time to prepare for that eventuality," he said.
"My officers had no option but to respond into the area to deal with protecting property."
Earlier, the Apprentice Boys and the nationalist Bogside Residents' Group appealed for trouble-makers to stay away from the parades.
The first parade began with 600 members of the local order, accompanied by six bands, making their way around the city's historic walls.
'Laid siege'
After the one-mile circuit, they attended a wreath-laying ceremony at the Diamond.
Following a religious service in St Columb's Cathedral, there was be a re-enactment of the Siege of Derry.
The governor of the Apprentice Boys had earlier called on those intent on causing trouble to stay away from the demonstration.
William Allen said it was important for the whole city that the celebrations were peaceful.
The Apprentice Boys parade commemorates the actions of Protestant Apprentice Boys who shut the city gates against the forces of the Catholic King James in December 1688.
King James laid siege to the city from December to August 1689 until the Protestant forces of King William of Orange relieved the city.
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