People's skulls were crushed with wooden clubs and metal spikes, defenceless women stripped and beaten and people were pulled off motorbikes and hit repeatedly with bamboo poles in the premeditated assault on May 30, Wunna Maung and Khin Zaw said.
"I saw with my own eyes the attackers striking down the victims with all their force and stabbing viciously with pointed iron rods," Mr Khin Zaw, 50, a senior executive of Ms Suu Kyi's party, the National League for Democracy, told the Thai senate foreign relations committee.
"Truly, it was a murderous attack. The beating was done until the victims died."
By the end of the two-hour attack near the village of Depayin in northern Burma the road was clogged with hundreds of bodies interspersed with pools of blood and scraps of clothing, said the two men, who entered Thailand illegally.
After the hearing they gave more details to journalists as they fled to the UN compound in Bangkok after being told that the Thai prime minister, Thaksin Shinawatra, wanted them arrested.
Yesterday they were in hiding with friends.
They could not say if Ms Suu Kyi, who escaped the scene but was detained a few miles down the road, was injured.
Mr Wunna Maung, 26, an NLD regional youth leader, said he saw the windows of her car smashed and bamboo poles thrown into the vehicle.
Ms Suu Kyi and dozens of her followers remain in detention despite protests by the international community.
The military junta, who say Ms Suu Kyi instigated the incident, refuses to release her until "the situation returns to normal".
The generals say only four people were killed but the witnesses' accounts tally with the findings of two American diplomats who visited the scene a few days after the attack.
Mr Khin Zaw said there were about 700 attackers initially but they were reinforced by about 2,000 more.
"They mainly aimed and struck on the head," he said. "Even when I was at a 100 yards I heard with anguishing pain, the popping sounds of heads being broken by savage blows."
Mr Wunna said he escaped uninjured because he was at the front of the convoy and the attack began at the rear, at about 8pm."They beat so severely it was as if people were crushed into pieces," he said.
"The women's shirts and earrings were stripped off them. I saw a young woman whose shirt and sarong was stripped off. They pulled their hair and crushed their heads on the road and beat them so severely.
"Scarcely no one was uninjured. I saw bloodstained people fall down and they were continued to be beaten after that."
At about 10pm he saw soldiers arrive.
"I heard about 10 shots and our vehicles were set on fire," he said.
After hiding for two nights in paddy fields he was helped by some villagers to meet about 100 of his NLD colleagues.
He said the villagers told him they saw soldiers take away about 80 corpses on trucks the day after the ambush.
The fleeing NLD members were joined by some of the attackers, who said they had been forced to join in and had been paid less than 50 pence a day.
Burma has opened a diplomatic offensive to stem international protests.
Khin Maung Win, the deputy foreign minister, went to Tokyo yesterday with photographs of a healthy looking Ms Suu Kyi to appeal to the Japanese government not to carry out its threat to freeze aid if she is not released.
The Japanese foreign minister, Yoriko Kawaguchi, rejected the request.
Members of the Burmese government in exile, which won the 1990 election but was not allowed to take office, are urging the UN security council to act.
They released a report yesterday on the May 30 incident, describing it as a "premeditated attack" which became a massacre, and said that the situation was worse now than in 1988, when the current junta seized power.
