CAIRO, Nov 30 (Reuters) - A top aide to Egypt's President Mohamed Mursi, who quit when the leader issued a decree expanding his powers, has joined the country's biggest opposition movement, a senior opposition figure said on Friday.

Samir Morkos was Mursi's adviser on the transition to democracy and the only Christian in the Islamist leader's team.

He is one of at least two presidential advisers to have stepped down since Mursi issued the decree on Nov. 22 that made his decisions immune to legal challenge.

The president's opponents see the move as a power grab that threatens Egypt's nascent democracy.

Morkos only learned the details of the decree when it was read out on state television, he told the London-based Asharq al-Awsat newspaper this week.

Without waiting for Mursi to approve his resignation, Morkos joined the National Salvation Front "and is now sitting beside me in a meeting," opposition leader Ahmed Saed told Reuters.

Morkos did not answer phone calls seeking comment.

The Front has led calls for protests against the decree and tens of thousands gathered for a rally in Cairo's Tahrir Square on Friday, some chanting "The people want to bring down the regime".

The opposition movement, which includes former Arab League Chief Amr Moussa and opposition figure Mohamed ElBaradei, has called for broader civil disobedience to fight what it describes as an attempt to "kidnap Egypt from its people".

Mursi says the decree is designed to speed up the democratic transition and will lapse as soon as Egyptians vote on a new constitution.

In an attempt to end the crisis, an Islamist-led assembly raced through approval of the constitution on Friday and plans to send it to Mursi for him to ratify and put to a popular vote. (Reporting by Yasmine Saleh; Editing by Tom Pfeiffer)