Six Celtic fans have been arrested following trouble on a plane, which led to an emergency landing at Cardiff airport.

The violence - described as a "riot" by the air crew - broke out on a charter Boeing 737 returning 148 fans to Glasgow from northern Spain, after the team's Uefa Cup game against Celta Vigo.

Some fans have complained the pilot over-reacted, but aviation officials and the police said the incident had been "serious".

Charter airline company Astraeus said the trouble began when a passenger started smoking in the toilets.

'Shocked and tearful'

When a flight attendant asked him not to: "The passenger objected... a number of other passengers became involved and a member of the cabin crew was struck on the arm."

The pilot issued a "mayday" distress call and diverted to Cardiff, and an RAF Sea King helicopter was scrambled.

I have seen the cabin crew and they were extremely traumatised, of that there is no doubt Superintendent Colin Jones

Armed police, firefighters and ambulance teams were waiting at Cardiff airport, which had swung into "full emergency" mode, when the flight landed.

Police took several passengers away for questioning.

Six men from Glasgow, aged between 37 and 51, were arrested for criminal damage, public disorder and assault.

Superintendent Colin Jones said further arrests could not be ruled out, and some people could be charged with more serious offences under the Aviation Act.

Cabin crew were shocked and tearful when he met them, he said.

"I have seen the cabin crew and they were extremely traumatised, of that there is no doubt," he said.

"My conjecture is that there was a serious incident on board that aircraft."

'Out of proportion'

He said there had been a "certain amount of damage" to the plane, and the incident must have been "shocking and frightening".

There was a discussion between one of the passengers and the cabin crew - it was a little bit heated but not too bad Passenger Steve Prince

He said although he did not know for sure how many fans had been involved in the trouble, he was confident the "main protagonists" were in custody.

But one fan said the incident had been "blown out of proportion".

According to passenger Steve Prince, the problem only began when the pilot told passengers they would be detained on the runway at Glasgow.

"After that, there was a discussion between one of the passengers and the cabin crew - it was a little bit heated but not too bad.

"One of the passengers leaned over and tapped one of the cabin crew on the shoulder.

"The next thing, we were descending at a rapid rate and heading to Cardiff.

"I'm a little surprised to hear there had been a riot on board. There were not any fierce postures taken."

Club ban

But Astraeus said the captain had made "absolutely the right decision" in diverting to Cardiff.

Armed police met fans coming off the plane

"Unruliness on the ground is one thing, but any sort of disturbance at altitude in a passenger aircraft is an extremely serious matter," it said.

Celtic Football Club said any fans convicted would be banned from future international travel and the club's ground.

"These individuals will have clearly undermined the reputation of the Celtic support. Such action will not be tolerated by Celtic," said chief executive Ian McLeod.

The fans not detained by police were put on a bus to complete their journey to Glasgow, and were due to arrive later on Saturday morning.