Egyptian authorities have prevented a leading political dissident from traveling to the U.S., saying Friday that the conditions of his early release from prison do not permit him to travel abroad.

Ayman Nour, freed on Feb. 18 for health reasons after nearly four years in prison, said a court ruled in July that his release should not carry any travel restrictions and that the authorities' decision was another attempt to stifle his political activity.

Nour, who challenged the country's longtime president, Hosni Mubarak, in a 2005 election, was jailed after being convicted of forgery in the vote. He and his supporters say the charges were fabricated to eliminate him from politics.

Nour was to travel Saturday to the United States, where he said he planned to give lectures about democracy and meet with lawmakers as well as former President Jimmy Carter.

"I am being strangled and politically assassinated," Nour said of the decision to bar him from making the trip. "They (authorities) don't want me to reveal to the West that a reformist political alternative exists in Egypt."

A police official said Egypt's chief prosecutor rejected his travel request because the conditions of his early release from a seven-year sentence do not allow him to leave Egypt. He spoke on condition of anonymity because he is not allowed to speak to reporters.

But Nour said he is free to travel and that he made a trip to Brussels in March to address the European Parliament that was approved by the prosecutor.

Nour, the leader of the Ghad party and the first politician to pose a credible challenge to Mubarak, said he would not be deterred and that he planned to try to run for president again in 2011, though it was unclear if election officials would allow him to contest the vote.

Egypt, a major U.S. ally and recipient of billions annually in American aid, has been ruled by Mubarak for 28 years. He has not yet said if he will run for another six-year term. It is widely believed that he is grooming his son to assume the presidency when he does decide to step aside.

Nour finished a distant second in the 2005 vote, Egypt's first contested presidential election.

At the time, Mubarak's government was under domestic and international pressure to enact political reforms, and Nour stepped forward as a charismatic voice calling for greater democracy.

In Washington, State Department spokesman Ian Kelly said the U.S. government was disappointed by the decision to bar Nour from traveling.

"We hope the government of Egypt will review its decision in this case and allow Mr. Nour to travel to the United States as planned," Kelly said.