|The Red Shirts' hinterlands are in northern Thailand, where the ruling party is struggling to make gains [Reuters]
The "Red Shirt" protesters are supporters of Thaksin Shinawatra, the prime minister ousted in a 2006 bloodless coup.
The group is centred around a body known as The United Front of Democracy Against Dictatorship (UDD), and has the support of several prominent academics and social activists.
The group was formed in 2008, as a counter to the yellow-shirted anti-Thaksin People's Alliance for Democracy (PAD).
The Red Shirts want the current prime minister, Abhisit Vejjajiva, to resign and call elections, saying that his rise to power was illegitimate.
They also accuse members of the Thai elite, particularly the military and judiciary, of undermining democracy.
Like Thaksin, the Red Shirts draw their support from the poorest sections of society, in particular the north and northeast of Thailand.
Since the 2006 ousting of Thaksin Shinawatra, Thailand's former prime minister, the country has been gripped by waves of political protest, some of it violent.
Here we take a look at a group that has emerged as one of the key players in the turbulent Thai political scene - the so-called Red Shirts.
Who are the Red Shirts?
Formally known as the UDD, Thailand's Red Shirt protesters first emerged as a political force in 2008.
Most, though not all, are staunch supporters of Thaksin Shinawatra, the former prime minister ousted in a 2006 bloodless military coup.
Thaksin himself, who has lived in exile since the coup, frequently addresses mass rallies of supporters via video link, statements on pro-Red Shirt websites, blogs and via Twitter.
To rally its cause and raise funds, the UDD operates dozens of community radio stations and a TV Channel, as well as a network of Red-Shirt merchandise shops.
It also claims to have around 400 regional organisations running "UDD politics schools".
Like Thaksin, the group draws the bulk of its support for the rural north and northeast of the country, although it also has backing from student groups and other activists.
Many of Thailand's rural poor benefitted from Thaksin's populist policies during his five years in power.
What do the Red Shirts want?
The Red Shirts say the current government of Abhisit Vejajiva, the Thai prime minister, is a puppet of the military and the Thai elite and came to power illegitimately.
They want nothing less than for Abhisit to resign and call fresh elections, saying his government has deprived them of their democratic rights.
The Red Shirts say their campaign is a fight against the political dominance of the unelected Thai elite – including royalists, top businessmen, the judiciary and senior generals – who they say have conspired to corrupt democracy and overthrow elected governments.
Abhisit became prime minister in December 2008 after a Constitutional Court ruling removed the pro-Thaksin People Power Party from power, saying its leaders had committed electoral fraud.
The ruling cleared the way for parliamentary manoeuvrings that allowed Abhisit's Democrat Party to take power without holding an election.
The next general election is not due to be held until 2011.
What is the Red Shirts' relationship to the Yellow Shirts?
The anti-Thaksin Yellow Shirts, formally known as the People's Alliance for Democracy (PAD), are the firm opponents of the Red Shirts.
Although the PAD has no official ties to Abhisit's Democratshas it does have a close relationship to key elements within the ruling party and says it is the true defender of Thailand's constitutional monarchy.
Yellow is traditionally a colour associated with Thailand's revered monarch.
The PAD draws its support from Thai royalists, businessmen and the urban middle class and accuses Thaksin of corruption, greed and nepotism during his time in power.
A series of anti-Thaksin street protests by the PAD in 2006 set the stage for the military coup that removed him from power.
What about Thaksin himself?
Aside from a brief return to Thailand in late 2007, the ousted prime minister has lived in exile since his ouster in the 2006 coup, rallying supporters by video links and other messages.
He has spent most time living in Dubai pursuing business deals and has also acquired passports from Montenegro and Nicaragua.
In 2009 he was appointed as a special economic advisor to the Cambodian government, further straining Thailand's already tense relations with Cambodia over a disputed border temple complex.
In 2008 Thaksin was sentenced in absentia to two years in jail after being found guilty of abuse of power in a land acquisition deal during his time in office.
Thaksin and his then wife, who had earlier returned to Thailand vowing to clear their names, had jumped bail two months previously saying they did not believe they would receive a fair trial.
Late last year Red Shirt leaders submitted a petition with 3.5 million signatures to the Thai king asking for Thaksin to be pardoned.