FACTBOX: Thailand's PAD, who are they?

(Reuters) - Flights were canceled at Thailand's main airport on Tuesday after anti-government protesters stormed the terminal building, leaving thousands of tourists stranded and threatening the country's tourism industry.

The move came after members of the People's Alliance for Democracy (PAD) breached police lines and roamed through the sprawling Bangkok terminal as startled tourists looked on.

Following are key facts about the PAD, a motley group of royalist businessmen, academics and a retired major-general:

* Founded in September 2005 by media proprietor Sondhi Limthongkul, a disgruntled former business associate of then-prime minister Thaskin Shinawatra.

Sondhi appeared in a Bangkok park to rail against Thaksin, accusing him of abusing his large parliamentary majority to further the business interests of his family and friends.

* The PAD swelled into a major anti-Thaksin street movement, especially when it hooked up with Major-General Chamlong Srimuang, an ascetic Buddhist who led a successful "people power" uprising against military rule in 1992.

Its protests were key to the political turmoil that led ultimately to the 2006 coup against Thaksin. It has been targeting the current government since May 25, saying the government is an illegitimate Thaksin proxy.

* The PAD's main draw card has been defense of the monarchy and 80-year-old King Bhumibol Adulyadej, regarded as semi-divine by many Thais, in the face of what they say is a bid by the Thaksin camp to turn Thailand into a republic.

PAD protesters are routinely clad in yellow. In Thailand every day is marked by a different color and yellow is the color for Monday, the day on which the King was born.

Thaksin and his supporters deny any challenge to the throne. * There have been question marks over the PAD's motives and backers. Its contempt for the results of three elections comfortably won by Thaksin or his allies has led to suggestions it represents neither the people nor democracy. PAD says 70 percent of MPs should be appointed rather than elected.

The alliance says it is funded by public donations. Analysts suspect it is also bankrolled by anti-Thaksin business interests, parts of the army or even factions within the palace.

* It became something of a social phenomenon, stretching way beyond the rock-concert-like stage where they held daily rallies on a major Bangkok intersection.

THe PAD has a radio station, satellite TV channel, several sympathetic newspaper titles and a slick, popular website.

It has also shown an uncanny ability to keep itself in the public eye and drive the domestic political agenda, jumping on issues such as a dispute with Cambodia over a 900-year-old Hindu temple to whip up anger against the government.

(Writing by Ed Cropley; Editing by John Chalmers)