"It is not possible to carry out a just, serious, clear and transparent investigation if the heads of the agencies remain in their place," leading Druze and opposition figure Walid Jumblatt said at his mountain residence in Beirut on Saturday.
"We warned against a security state over and over."
A UN fact-finding team said in a report released on Thursday that Lebanon's inquiry into al-Hariri's 14 February killing was seriously flawed and called for an international investigation, long a demand of the opposition that holds Damascus and the security services it backs responsible.
But the report said even an international inquiry was unlikely to carry out its tasks satisfactorily while the Lebanese security chiefs stayed.
Syria denies involvement in al-Hariri's assassination, which has plunged Lebanon into its biggest political crisis since the end of the 1975-1990 civil war.
Lebanon's pro-Syrian authorities have rejected the UN report.
But Lebanon's Christian opposition echoed Jumblatt's comments, saying the report bolstered its repeated calls for an international investigation and for the security chiefs to go.
"It should place itself at the disposal of the international investigative committee we call for..."
The Christian opposition also took aim at the outgoing pro-Syrian government, which stepped down last month under international and domestic pressure.
"The resigned government, especially those ministers charged with security and judicial affairs, has moved ... to the position of accused because of its direct political responsibility," it said in a statement.
"It should place itself at the disposal of the international investigative committee we call for and stop the manoeuvres some ministers have made in a desperate campaign of self-defence," the statement said.
Lebanese member of parliament Basim Yammut told Aljazeera that the findings of the report were important for maintaining the country's unity.
Yammut echoed the demand for removing heads of the security services or suspending them until the international investigation is complete.
Syrian troops move out
Also on Saturday, Syrian troops pulled out of the Majdaleon area, west of Baalback in the Bekaa Valley, Aljazeera's correspondent in Lebanon reported.
The troops returned to Syria and were headed towards the city of Homs.