VIOLENCE erupted when Bolton Wanderers fans entered the rival fans' enclosure during the Lancashire derby against Blackburn Rovers.

More than 10 Whites supporters unveiled a flag after getting into the CIS stand at Blackburns Ewood Park.

The stunt sparked a number of scuffles among fans and led to a steward being punched and head-butted as violence flared.

Inspector Andrea Bradbury, of Blackburn Police, said: "A group of four Bolton Wanderers supporters who infiltrated the CIS Stand were arrested for assault and violent disorder.

"A steward at the game was punched and head-butted during the incident.

"A total of 13 Bolton fans were arrested for drunkenness or public order offences.

"We have no reports of any trouble after the game."

The game between the Lancashire rivals - which has been plagued by problems with violence over the years - finished 0-0, but saw Boltons Hidetoshi Nakata sent off after being given two yellow cards for bookable offences, just after the half-hour mark.

And there were eight yellow cards, six of them to Bolton players.

Match referee Mike Riley was criticised by both managers, with Bolton boss Sam Allardyce claiming the Leeds official could have caused a riot because of the way he handled the derby match.

After the game, Allardyce claimed the trouble in the CIS Stand had been caused by the referee.

He said: "The referee nearly caused a riot. Some of our fans were over the fence and had a go at their supporters, and that wasnt because of the players."

The nine men arrested for public disorder were given on-the-spot fines.

Four men arrested on suspicion of assault and violent disorder have been bailed pending further inquiries.