The United Front for Democracy Against Dictatorship (UDD; Thai: แนวร่วมประชาธิปไตยต่อต้านเผด็จการแห่งชาติ; นปช., alternatively translated as National Democratic Alliance against Dictatorship), whose supporters are commonly called Red Shirts, is a political pressure group opposed to the People's Alliance for Democracy (PAD), the 2006 Thai coup d'état, and supporters of the coup.

Notable UDD leaders include Jatuporn Prompan, Nattawut Saikua, Veera Musikapong, Jaran Ditapichai, and Weng Tojirakarn.

Before the July 2011 national elections, the UDD claimed that Abhisit Vejjajiva's government took power illegitimately, backed by the Thai Army and the judiciary.

UDD accused the country's extra-democratic elite—the military, judiciary, certain members of the privy council, and other unelected officials—of undermining democracy by interfering in politics.

In response to violent anti-government PAD rallies and the yellow shirt seizure of Government House in May 2008, the UDD organized counter-demonstrations, which would sporadically result in injuries and deaths.

After Prime Minister Abhisit Vejjajiva took office, the UDD led major anti-government rallies in April 2009 and March–May 2010, leading to violent clashes with military forces.

The UDD calls for the replacement of the Ammatayathipatai (อำมาตยาธิปไตย, aristocratic polity)—the system in which palace insiders, the military, and bureaucrats effectively ignore the popular mandate—with an electoral democracy.

Before daybreak on 2 September 2008, UDD supporters rallying at Sanamluang attacked PAD demonstrators gathering on the Makkhawan Rangsan Bridge at Ratchadamnoen Klang Avenue.

In March 2009, Thaksin Shinawatra claimed via a video broadcast to a UDD rally that Privy Council President Prem Tinsulanonda masterminded the 2006 military coup, and that Prem and fellow Privy Councillor members Surayud Chulanont and Chanchai Likhitjittha conspired with the military and other members of the "Ammatayathipatai" (อำมาตยาธิปไตย—aristocratic polity) to ensure that Abhisit became premier.

On 11 April 2009, violent clashes occurred between the UDD and blue-shirted government supporters brought in by coalition partner Newin Chidchob.

[7] The protests caused the summit to be cancelled, leading Abhisit to declare a state of emergency in the areas of Pattaya and Chonburi on 11 April after his vehicle was attacked and damaged by UDD demonstrators.

Legislation authorizing emergency decrees was originally pushed through parliament in 2005 by the Thaksin government, provoking charges of authoritarianism at the time by Abhisit.

[11] At a demonstration in front of Prem's residence, a yellow shirted driver plunged her car into a crowd of UDD protesters injuring several before driving away.

[14] In the pre-dawn of Monday, 13 April, soldiers used tear gas and fired live and training rounds to clear protesters from the Din Daeng intersection near the Victory Monument in central Bangkok, injuring at least 70 people.

[30] Prior to a gathering of the red shirts planned for 30 August 2009, the prime minister approved the establishment of the Center for the Administration of Peace and Order (CAPO) (ศูนย์อำนวยการแก้ไขสถานการณ์ฉุกเฉิน (ศอฉ.)

A government/military situation room for monitoring the protest, officially called the Peacekeeping Operations Command, was established at the 11th Infantry Regiment in Bang Khen.

Suthep claimed that the UDD protesters planned to "besiege government offices and residences of important figures, like Privy Council President Prem Tinsulanonda.

On Friday, 12 March, police and military checkpoints were set up along all main routes leading to Bangkok to inspect UDD protesters for weapons being carried into the capital.

[49][50] On 28 April, Thai security forces and anti-government protesters clashed on the outskirts of Bangkok, with troops firing both over and then directly into a crowd of red shirts to keep them from expanding their demonstrations.

[citation needed] Jatuporn Prompan, Nattawut Saikua and other Red Shirt leaders surrendered to police, according to them, to prevent further bloodshed during the military crackdown on 19 May 2010.

CAPO (Center for the Administration of Peace and Order), first established in August 2009, was re-established after the cabinet approved the imposition of Chapter 2 of the Internal Security Act (ISA) in seven districts of Bangkok from 9–23 February to control the rallies by the yellow shirt People's Alliance for Democracy (PAD) and the red shirt United Front for Democracy against Dictatorship (UDD),[52] to which local reaction to the Cambodian–Thai border dispute forms a background.

[53] PAD (yellow shirt) leaders Sondhi Limthongkul, Chamlong Srimuang, and others were indicted on 27 December for storming the prime minister's office compound and sealing off parliament during the massive anti-government rallies in 2008.