Youths have been grilled by police after hurling bricks from 120ft high Hewenden Viaduct on the just-opened Great Northern Trail, at Cullingworth.

They were caught by farmer James Lawn, of East Manywells Farm, Cullingworth, and his neighbour, John Griffith, after a chase across fields.

Mr Lawn saw youths standing on the parapet and throwing stones on to cattle below, on Monday evening.

Two of the gang were caught hiding in bushes near Hallas Bridge after a chase and "frogmarched" back to Harecroft where they were handed over to police.

Mr Lawn said he wished to praise police for their swift arrival on the scene, shortly after 8.30pm.

"Trouble like this needs nipping in the bud," said Mr Lawn.

"It's a lovely walk and it was glorious on Monday, with the blue sky and birds singing, but it had to be spoilt by these idiots. I hope this is the last time this happens."

He spotted the gang of about eight youngsters in their mid teens standing on top of the parapet.

"I saw them raining down stones on the cattle below," he said.

Mr Lawn telephoned a friend and together they chased two of the youths while the police were called.

"We caught them hiding in bushes at Hallas Bridge and frogmarched them back to Harecroft to where the police were," he added.

David Stevens, of Sustrans, the alternative transport group which is planning to open up the rest of the trail as far as Queensbury, said: "We can't let a few youths spoil it for hundreds of other users.

"This type of behaviour needs stopping straight away. We are very grateful for these people and the police acting so quickly."

Sustrans officers were also very concerned that youngsters were facing danger by climbing on to the parapet.

"We feel climbing on the parapet is very reckless," he added.

He said the trail was already proving very popular and had attracted hundreds of people in the two weeks it had been open.

In a single hour last Sunday a ranger had counted 200 people using the trail, he said.

A Bradford police spokesman said youths were reported throwing bricks on to cows and calves from the viaduct at 8.45 on Monday evening.

"Two juveniles were returned home and their parents informed of what they had been doing, " he said.

The incident would also be reported to the neighbourhood policing group to follow up.

The Great Northern Trail is a cycleway, footpath and bridleway, which will run between Cullingworth and Queensbury and is being constructed in three sections. Sustrans estimates the cost of the project will be £1 million.