LYON, France - Fans poured onto the pitch at Thursday's Europa League clash between Lyon and Besiktas, delaying the start of a violent, high-risk match that delivered another body blow to European soccer.

The start of the quarter-final first leg was delayed by 45 minutes as officials attempted to clear the pitch at Parc OL. Many fans had already run a gauntlet of violence outside the ground, where police used tear gas to quell outbreaks of fighting and vandalism.

The incidents were the latest in a black week for European club soccer after the Borussia Dortmund team bus was targeted by roadside bombs just before Tuesday's Champions League game at home to Monaco.

In the aftermath of that incident, which postponed the match 24 hours, UEFA demanded a re-evaluation of security measures "wherever necessary".

According to an AFP reporter at the Lyon ground, home supporters poured onto the pitch as they attempted to protect themselves from firecrackers and other objects raining down from the stands.

Lyon president Jean-Michel Aulas was seen calling for calm inside the stadium via a loudspeaker.

As the match - which Lyon won 2-1 - progressed, blame for the violence was already being apportioned.

Turkish President Recep Tayyip Erdogan, who faces a referendum on Sunday which could expand his powers, told NetTV: "If the French (fans) went onto the pitch, then that's very dangerous."

One Lyon fan, who gave his name as Amin, said: "It is the first time I have ever seen anything like this here."

Earlier, Turkish fans clashed with security forces, leaving a stadium steward injured.

"The Turkish supporters charged at a steward and injured him," said an AFP photographer who witnessed the incident. "The supporters were waving big Turkish flags."

Trouble also erupted between rival fans outside another entrance to the stadium as smoke billowed from tear gas canisters.

A group of Turkish fans from Germany smashed a pair of glass doors at the club megastore after they were unable to purchase tickets to the sold-out match.

The pro-government Turkish media blamed French fans for the trouble.

"The Lyon hooligans attacked children," said the Yeni Safal daily newspaper, with a picture it said showed a French fan waving a crutch in the direction of a child.

Sabah, another pro-government daily website, claimed: "Lyon hooligans attacked the Turkish fans, including women and children, and afterwards the Besiktas fans responded."

Authorities had categorized the match as "maximum risk", with 1,000 stewards drafted in for the encounter.

Around 15,000-20,000 Turkish fans had been expected for the game taking place at a stadium situated in an area of the city which is home to many people of Armenian origin.

Agencie France-Presse