Every year Berlin observes its own very peculiar tradition. On May 1 -- International Workers Day -- its citizens brace themselves for a day of rioting and mayhem. And this year was no different.

Many Berliners had been hoping that this year's May Day would pass off peacefully, that the idea of a ritual riot had become just a little old hat. And for most of the day it seemed it might actually happen: there were family-friendly street parties and peaceful left-wing demonstrations.

But once night fell, the revolutionary habit was back: gangs of rioters threw stones and bottles at the police and garbage bins were set alight. Several people, including police officers, were injured and there were 115 arrests.

Nevertheless, Berlin's police chief Dieter Glietsch said that he was pleased with how the operation had gone this year, pointing out that there was a lot less violence than in previous years.

Earlier in the day, around 50,000 Berliners had gathered to enjoy the "Myfest" street party in the Kreuzberg district, which included concerts and various events for children. The festival had been organized by local government, residents and local businesses to try to create an alternative to the traditional revolutionary antics. Two left-wing demonstrations with around 6,000 participants also passed off peacefully.

But once the party was over and the stages had emptied, around 600 people decided there was nothing better to do than have a riot. And while the police initially held back in order to prevent the whole thing from escalating, they eventually began to pick off the ringleaders and arrest them. Still, by midnight the revolution was over.

A tradition is born

The rioters were "celebrating" the 20th anniversary of the first major riot in Kreuzberg in 1987. The district, which belonged to the western part of the then-divided Berlin, was a kind of sub-culture enclave with punks, squatters and assorted revolutionaries living side by side with a large Turkish community. On that first May Day over 900 rioters fought running battles with the police for 12 hours. Shops were looted and cars were set on fire -- and a ritual was born that would be reenacted to a greater or lesser extent over the next 20 years.

In recent years, the authorities have revised the way they deal with the disturbances: Instead of heavy-handed tactics, they now employ a more softly-softly approach, using "anti-conflict" teams to keep the peace and imposing bans on glass bottles and cans at the street parties.

Still, this year the police had been prepared for trouble, and there were 5,000 officers deployed to keep the peace. There were concerns that some left-wing groups might use May Day as a dress rehearsal for protests against the upcoming G8 summit  at Heiligendamm in June. And already on Monday night, on the so-called Witches Night, there had been some disturbances  in the eastern district of Friedrichshain. A riot broke out after a punk concert, during there were 119 arrests and 15 police officers were injured.