* Liam Gallagher kick starts music at Glastonbury 2013
* Arctic Monkey, Rolling Stones, Mumford & Sons headline acts
* Downpour makes site a mud pit for soaked revellers
By Belinda Goldsmith
PILTON, England, June 28 (Reuters) - British rocker Liam Gallagher kicked off three days of music at the Glastonbury festival on Friday, cheered on by mud-splattered revellers glad to see the skies clear after 18 hours of rain.
In one of the worst-kept secrets of the 2013 festival, Gallagher and his band Beady Eye opened the day at 11 a.m. (1000 GMT) from one of 58 stages scattered across a sprawling 900 acre (365-hectare) site at Worthy Farm in rural southwest England.
Despite vowing never to play at Glastonbury again after criticising the event in 2004, the former Oasis frontman kicked off an hour-long set with "Flick of the Finger" and pulled out crowd pleasers like "Morning Glory".
"It's never too early for a bit of rock 'n roll aggro, is it?" a black-clad Gallagher told the heaving crowd, wearing a pair of sunglasses.
Other acts on the bill for Friday include Sinead O'Connor, Dizzee Rascal, Rita Ora, and headliners Arctic Monkeys, while the year's main act, the Rolling Stones, will play on Saturday and British folk rockers Mumford & Sons on Sunday.
More than 135,000 fans have descended on the farm located about 130 miles (210 km) southwest of London for Britain's largest music festival, which started off as a retreat for about 1,500 hippies in 1970 who paid one pound and got free milk.
True to Glastonbury's alternative roots, the festival includes music of all genres, from hip hop to chanting monks, and surprising choices like U.S. country star Kenny Rogers and octogenarian British TV presenter Bruce Forsyth.
Solange Knowles, younger sister to 2011 headliner Beyonce, will play one of the smaller stages on Friday.
The festival was not held last year due to the London Olympics and demand for the 205 pounds ($315) tickets was strong, selling out in a record one hour and 40 minutes.
While Glastonbury is known for its megastars, it also has a reputation for mud, and this year proved no exception with campers drenched on Thursday and some creating mud slides. Forecasters expected the rest of the weekend to stay dry.
Although the main music programme only started on Friday, revellers have been arriving since gates opened early on Wednesday, keen to secure the best camping spots.
For those unwilling to sleep in a normal tent, there are up to 1,000 tipis and Mongolian-style tents called yurts available for hire, which are set up in advance and can come with furnishing depending on price level.
Tara Weightman, managing director of company Hearthworks that oversees the ready-pitched accommodation, said a record number of 10,000 people had paid to camp in style this year, while separate VIP areas catered for the musicians.
The Rolling Stones' frontman Mick Jagger tweeted that he would be staying in a yurt, but the location remains unknown.
"It is popular as it makes it so much easier for people as they don't have to come early to set up camp and they arrive to find it all ready for them," Weightman told Reuters.
One happy customer was Steve Pratt, 29, an insurance agent, who paid 950 pounds to share a tipi with four friends in a designated areas where toilet facilities were less crowded.
"I've been to Glastonbury before with my own tent but this really appealed to me as it gives you more space and some quiet - and you can escape the rain in comfort," Pratt said. (Reporting by Belinda Goldsmith; editing by Mike Collett-White)