The probe blamed both the red shirts and the military for the violence that resulted in at least 92 deaths.
The commission also shed more light on the role of the "men in black" in deadly attacks during the protests, confirming their links to red-shirt figure Maj Gen Khattiya Sawatdiphol, known as Seh Daeng, and the protesters' security guards.
Somchai Homla-or, the fact-finding sub-committee's chairman, spent an hour addressing a press conference to present the TRC's report.
He said the paper should be treated as the most reliable and credible record of the events, although more facts could have been discovered if the power of the commission had not been limited.
The report, nearly 400 pages in total, was released to conclude the two-year term of the TRC, whose chairman was appointed by former prime minister Abhisit Vejjajiva.
Australian and Swiss ambassadors, along with a dozen foreign diplomats, members of international organisations such as the UN Office of Human Rights and Human Rights Watch and members of red and yellow shirt movements were present at the TRC press conference.
Mr Somchai concluded that the 2010 violence was the result of a series of conflicts which began during the government of former prime minister Thaksin Shinawatra, the coup in 2006 and anti-government protests in Pattaya and Din Daeng during Songkran in 2009.
He claimed the TRC has found evidence that the men in black who engaged in violent attacks on the authorities using M79 grenades and rifles had received support from red-shirt security guards linked to Maj Gen Khattiya.
The attacks caused nine deaths, killing a member of the Love Silom Group along with two policemen and six military officers, including Gen Romklao Thuwatham, at Khok Wua intersection on Ratchadamnoen Avenue on April 10, 2010.
The officer was leading a unit of troops to clear red-shirt protesters from a section of the avenue when he was hit by an M-67 grenade, believed to have been hurled from a wooden house opposite Satreewithaya School at the intersection.
A member of Maj Gen Khattiya's political party rejected Mr Somchai's statement that the officer, who was killed near the Lumpini MRT station on May 13, 2010, was seen on April 10, the same day the men in black were sighted.
Atipparut Garnjanasoowun, founding member and secretary-general of the Khattiya Party, said Maj Gen Khattiya had left the protest site on April 9 and it was impossible that he had been seen at the rally on April 10.
"This report has misstated information and dates and should be clarified, otherwise it will be perceived that Seh Daeng [Maj Gen Khattiya] was responsible for all the violence," he said.
Mr Somchai said nearly half the deaths took place during the government's campaign to reclaim protest zones.
Accounts from witnesses and other evidence also showed that authorities and the men in black had exchanged gunfire at Wat Pathum Wanaram, Mr Somchai said, adding that the six deaths at the temple were caused by shots from the direction in which security forces were stationed nearby.
While referring to the men in black, Mr Somchai said it was difficult to further identify who these people worked for.
The group's mission appeared to be to attack and make it harder for authorities to do their job during the protest.
Mr Somchai said the TRC's investigation also found that Maj Gen Khattiya was shot from a spot in an area under the control of the military.
It was unclear, however, which building the gunman had fired at him from.
The TRC concluded that the protest organiser _ the United Front for Democracy against Dictatorship (UDD) _ was partly to blame for the violence.
Protest leaders provoked demonstrators with their speeches and made inadequate efforts to prevent the violence, Mr Somchai said.
The government, meanwhile, failed to control the military's use of war weapons to quell the protest, he said.
The government-appointed Centre for the Resolution of the Emergency Situation did not have the capability to monitor and evaluate the implementation of the crowd control measures, Mr Somchai said.
Kittipong Kitayarak, permanent secretary of justice and TRC member, called on political groups to avoid exploiting specifics in the report for political gain.
Kanit na Nakorn, TRC chairman, said his personal observation was that former premier Thaksin Shinawatra should stay out of Thai politics.
However, Kwanravee Wangudom, of the People's Information Centre, said the TRC report was ambiguous and did not assign responsibility for the events which took place.
Army chief Gen Prayuth Chan-ocha said the TRC was not a court and investigations into the clashes should be conducted in court.
Pheu Thai Party list MP Weng Tojirakarn, who is also a red-shirt protest leader, yesterday demanded the TRC present evidence to back its conclusion that armed black-clad men were present at the rally and enjoyed support from his red-shirt group.
He rejected the TRC's claim that men in black fired at soldiers on an elevated electric train platform in Pathumwan, forcing soldiers to return fire.
Opposition and Democrat Party leader Abhisit welcomed the TRC findings.
"Prime Minister Yingluck Shinawatra said on several occasions that her government would accept the work of the TRC. Therefore, the TRC's recommendations should be considered," he said.TRC report highlights
- "Men in black" were supported by United Front for Democracy against Dictatorship (UDD) guards and were close to late Maj Gen Khattiya Sawatdiphol.
- The men in black were involved in the deaths of eight security officers, including Gen Romklao Thuwatham.
- Gen Romklao Thuwatham died as a result of injuries from M67 grenade shells, not bullets.
- UDD leaders failed to prevent violence.
- Soldiers used many live rounds in their operations.
- The Centre for the Resolution of the Emergency Situation (CRES) lacked a system to monitor the operations of officers.
- The CRES deployed a number of war weapons and equipment in its operations, which instigated distrust among protesters.