HAVANA - (AP) -- Fidel Castro on Thursday announced a petition campaign on a constitutional change that would declare the country's socialist system "untouchable.''

  Cubans will be able to sign petitions beginning on Saturday in support of Cuba's economic, political and social systems, Castro said during an appearance on state
  television.

  More than 120,000 petition stations will be set up around the island, he said.

  Castro said ''no compatriot will be denied the opportunity'' to register their support for the constitutional amendment proposed Monday by the island's popular support organizations, which are linked to the Communist Party of Cuba.

  The announcement came one day after Castro called out millions of people to march in hundreds of communities around the country in support of the constitutional
  proposal declaring that the system cannot be changed. Castro himself led the early morning march in Havana.

  Opposition activists say the government's appeals for public support of its socialist system are a response to the Varela Project, an initiative that would ask voters if they favor civil liberties including freedom of speech and assembly, the right to own a business, electoral reform and amnesty for political prisoners.

  Most Cubans first heard of the Varela Project in mid-May, when former President Jimmy Carter mentioned it in a live television address to the Cuban people. Carter suggested that the plan be published in the state media. But that has not happened.

  Varela Project activists on May 10 submitted more than 11,000 signatures to the National Assembly soliciting a referendum.

  Castro has said nothing publicly about the Varela Project or a referendum.

  More details about the new amendment and Saturday's petition drive will be given tonight on the government's nightly television program, Castro said.

  With ''the proverbial organizational ability acquired by our people, their culture, and their political conscience, this important step will be made, and at the end there will remain absolutely no doubt about how the people of Cuba feel and think,'' he said.

  ''Millions of Cubans will also give, in the same way, an appropriate and resounding response to the liberator who no one invited: Mr. W. Bush,'' Castro said of President Bush, using a shortened nickname he recently adopted.