"The national security authorities informed us that this rally... was banned for security reasons," the leftist Tagammu party's Husayn Abd al-Razik told a press conference on Wednesday in Cairo.
"The interior minister must resign since he cannot guarantee security for the planned rally, which would have brought only 1000 people," Abd al-Razik said.
The Tagammu, Nasirite and liberal Wafd parties, along with the banned communist party and six human rights groups, had planned to release a petition demanding radical political reform at their rally in Cairo on Wednesday.
"This rally is not cancelled: the organising committee is just delaying it," Abd al-Razik said, adding a new date would be set on 5 November and, if it were refused, the committee would file a complaint.
The petition calls for a reform of the constitution, limiting the tenure of a president to two terms while requiring that the head of state be elected by universal suffrage rather than by referendum.
"The interior minister must resign since he cannot guarantee security for the planned rally, which would have brought only 1000 people"
The petition that has been circulating since May has obtained "thousands of signatures of public figures, academics and members of professional associations," Abd al-Razik said.
Demonstrations are banned in Egypt under emergency laws in place since 1981, though they are tolerated on university campuses and in mosque compounds.
President Husni Mubarak, who will complete his fourth term in office in 2005, announced last month plans for democratisation, including updating laws governing the activities of political parties.
Mubarak's ruling National Democratic Party holds the majority of seats in parliament.