CAN you hear it? In the distance? It sounds like thunder. In the glens.

More than 5000 bikers are currently roaring their way through the highlands in the wonderfully named Thunder in the Glens rally.

Now in its 20th year, the three-day event - which ends today - has finally gone mainstream and boasts a record numbers of bikers.

Rally coordinator, George 'Mad Dog' McGuire, told the Sunday Herald that Thunder in the Glens was now "unparalleled" in size and is the biggest event of its kind in Europe, as he prepared to lead a mass ride-out of bikers from Aviemore to Grantown-on-Spey yesterday afternoon - the highlight of the rally.

A veteran of the rally, McGuire, who attended his first Thunder in the Glens in 1998, said the sight of the bikers blasting-off en masse through the Highlands and roaring through the scenic surroundings was breathtaking.

"It's amazing and it leaves a big lump in my throat. I have to fight back the tears", he said as thousands of Harley Davidson owners revved-up their engines, while a crowd of more than 1,000 loudly cheered.

McGuire added: "I'm passionate about it and just love it because it's a great event for Scotland. There are people of all ages and there is a great camaraderie."

The annual pilgrimage for Harley-Davidson owners and fans from across the world is organised by the Dunedin Chapter of the Harley Owners Group (HOG). The Scottish band Big Country launched proceedings on Friday night.

McGuire said: "The whole thing has totally blown us away and it's the biggest on record for us. The fact that it's the 20th anniversary means there are lost of people here who were on the first and early ones.

"It's brought home just what a unique and fantastic event it is with breathtaking scenery. It's an event that's unparalleled in Europe.

"Last year we had about 2,500 people, so to get it to 5,000 is amazing and beyond what anyone was expecting." There are people here this time who haven't come for years, but because it's the 20th year they have made the trip.

"We've also got a lot of first timers and there are lots and lots of youngsters alongside all these big hairy bikers.

"In recent years we've noticed that lots of bikers families come along, but this time a lot of the friends of the kids of bikers seem to be here too."

McGuire said this year's event was worth up to £14 million to the economy of the Highlands - about £4m higher than the usual annual benefit - due to the higher than expected attendance.

Bikers taking part in yesterday's ride-out - which took in the Cairngorms and stunned tourists with their noisy display - said the numbers attending the rally had surged since the the early years of the event, when it started out with just a few hundred diehards.

Douglas Milne, 57, a building contractor from Stonehaven, who was on the first ever Thunder in the Glens rally, said: "There are people who you don't see all year and then you see them here and it's like it was just yesterday.

"It's great company at the rally, and at the ride-off every year makes the hairs on the back of your neck stand up."

Sandy Scott, 71, a retired medical sales representative from Blairgowrie, is the road captain for the rally, who rode at the head of the bikers as they left Aviemore centre.

He said: "It's quite chaotic and fiery, with all the bikes and it's always buzzing. But it's brilliant for everyone as the atmosphere is electric and you get people from all parts of the UK, Europe and there are lots of families."