|The Red Shirts' core of support is in northern Thailand, where the ruling party is struggling to make gains [Reuters]
The "Red Shirts" are supporters of Thaksin Shinawatra, the former Thai 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. It was formed in 2008, as a counter-force to the yellow-shirted anti-Thaksin People's Alliance for Democracy (PAD). A series of anti-Thaksin street protests by the PAD in 2006 set the stage for the military coup that removed him from power.
The Red Shirts called on former prime minister Abhisit Vejjajiva to resign and call elections, saying that his rise to power was illegitimate. They have said 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.
Like Thaksin, the Red Shirts draw their support from the poorest sections of society, in particular Thailand's north and northeast.
Many of Thailand's rural poor benefitted from Thaksin's populist policies during his five years in power, although it also has backing from student groups and other activists.
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.
Thaksin, 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".
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 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.
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 late 2010, Red Shirt leaders submitted a petition with 3.5 million signatures to the Thai king asking for Thaksin to be pardoned.
Red Shirts have rallied in defence of embattled Prime Minister Yingluck Shinawatra - Thaksin's sister - in response to mass protests calling for her resignation that began in November 2013. The Red Shirts argue that Yingluck was democratically elected, and that demands for her to step down are illegitimate.