PKR communications director Fahmi Fadzil described the report of the British newspaper the Daily Mail as "wild allegations", Xinhua reported.
"If there is any concrete data backing up an allegation, we will respond to it. If anyone aboard (the flight) chose to attend the Anwar trial that is their right as it was in open court," he told English newspaper the Malaysian Star.
The Daily Mail report said that Ahmad commanded the flight several hours after he had attended the sodomy trial of opposition leader Anwar Ibrahim Friday.
Malaysia Airlines flight MH370 with 239 on board vanished mysteriously.
Ibrahim was jailed for five years last Friday but was released on bail.
Malaysia Airlines flight MH370 with 239 passengers and crew on board vanished mysteriously about an hour after taking off from Kuala Lumpur early morning March 8. The Boeing 777-200ER was initially presumed to have crashed off the Vietnamese coast in the South China Sea.
The plane lost contact along with its radar signal when it was flying over the air traffic control area of Ho Chi Minh City in Vietnam.
Asked if Zaharie Ahmad was often seen at functions organised by PKR or the opposition alliance Pakatan Rakyat (People's Alliance), he said he didn't not know Ahmad personally.
"I did not know who he was until the news broke out about the plane last Saturday," said Fahmi.
PKR vice president N. Surendran said there "is not a drop of evidence to support such speculation".
"Let us be fair to the missing captain as well -- he cannot answer for himself," he said.