In comments made to Egypt's El Osboa newspaper, al-Assad accused the Paris-based Abdel Halim Khaddam of scheming against Syria before resigning in June and insisted the allegations levelled by him was simply not true.
"This incident did not happen. The aim of spreading these allegations is to link the threat to the assassination. The game is over," al-Assad said, responding for the first time to Khaddam's accusations.
"I wish to say here that no one joined us in the last meeting between me and Hariri, so where did these allegations come from," he said in the interview.
Al-Assad also hinted that as president of Syria, he would be immune from questioning by a team investigating al-Hariri's February assassination.
Khaddam (L) has levelled serious
charges against al-Assad
The UN team probing the killing of al-Hariri has requested to meet al-Assad and Farouq al-Shara, the Syrian foreign minister.
Diplomats said on Wednesday Syria had agreed to let al-Shara be questioned and was considering its position over al-Assad.
Al-Assad said Syria had previously voiced its readiness to cooperate with the probe if the request had a "legal basis".
"This is not the first request. There was a previous request, when the committee proposed to come to Syria at the end of last summer to listen to Syrian witnesses, as they call them. At that time, they requested to meet with President Bashar, and the president of the republic has international immunity," he said in the interview with El Osboa's editor Mostafa Bakry.
Khaddam, who first attacked al-Assad in a television interview and accused al-Assad of threatening al-Hariri, renewed his assault in a newspaper interview published on on Friday.
Khaddam, who was a trusted aide of al-Assad's president father for about 30 years, told London-based Asharq al-Awsat he was working to topple the Syrian government.
"This incident did not happen. The aim of spreading these allegations is to link the threat to the assassination. The game is over"
When asked whether Khaddam was working against Syria before he left office, al-Assad told El Osboa: "In my view, he was involved with the plan before and he was a main player in it, but we don't have specific details until now."