Mr Suthep was testifying for the tirst time in a lawsuit launched by him and Mr Abhisit, accusing Mr Tarit and four other DSI investigators with malfeasance.
The case is a counter-suit to DSI attempts to indict Mr Abhisit and Mr Suthep for premeditated murder and deadly assault in connection with the 2010 street violence in Bangkok.
Mr Suthep, as deputy prime minister, and Mr Abhisit, as prime minister, headed the Centre for the Resolution of the Emergency Situation, known as Cres, a special military-run unit to deal with the red-shirt protests.
In his testimony on Monday, Mr Suthep said armed elements among the red-shirt protesters attacked soldiers with war weapons at Kok Wua intersection on Ratchadamnoen Avenue in April 2010.
Several soldiers were killed and injured, Mr Suthep said. He ordered the soldiers to withdraw, but they were surrounded and attacked by the protesters.
He subsequently ordered them to use shotguns in self defence. The soldiers had been trained to shoot below the knees and keep a distance from the protesters, he said.
Mr Suthep testified he issued the order to shoot, and Mr Abhisit had nothing to do with it.
A month later, on May 19, when soldiers were closing in on the Ratchaprsasong rally site, unknown armed elements fired at them on Sarasin Road, killing soldiers and a foreign journalist, before the protest leaders surrendered.
Mr Suthep said these cases were handled by Mr Tarit, who then charged the red-shirt leaders with terrorism. The statements at the time said there were armed elements at the rally sites, as well as men in black who used war weapons against the soldiers.
The terrorism cases also are in court.
Later, after Yingluck Shinawatra became prime minister, Mr Tarit changed his story, Mr Suthep said.
He called Mr Suthep and Mr Abhisit in to hear charges of premeditated murder, but the facts he then presented were different from those he presented earlier in the red-shirt terrorism cases.
Mr Suthep told the court Mr Tarit defamed him and Mr Abhisit to ensure criminal charges were brought against them. He claimed Mr Tarit was using accusations about the use of force by soldiers while ignoring the fact that red shirts had armed elements and men in black amongst them.
Both the Truth for Reconciliation Commission of Thailand and the Human Rights Commission, a constitutional agency, had made findings that there were armed elements among the protesters, Mr Suthep said.