* April 6 harnessed social media for Arab Spring uprising
* Ruling stems from complaint April 6 conspired against Egypt
* Leading figures already jailed over protests (Recasts with details from court document)
CAIRO, April 28 (Reuters) - An Egyptian court on Monday banned a pro-democracy movement that helped ignite the uprising that toppled autocrat Hosni Mubarak in 2011, tightening a crackdown on dissent a month ahead of a presidential election the former army chief is expected to win.
The Cairo court was ruling on a private lawsuit brought by a lawyer who alleged the April 6 movement had conspired against Egypt to serve "foreign parties" and staged protests that had "insulted national security" and damaged the economy.
April 6 was one of the youth protest movements that harnessed social media to bring people into the streets for the historic Jan. 25, 2011 protests that led to Mubarak's downfall at the height of the "Arab Spring" uprisings.
It also took part in protests that led to the army overthrow of Islamist president Mohamed Mursi in July last year but has since mobilised opposition to the military-backed government, rallying against a law that tightly restricts street activism.
The "Cairo Court for Urgent Matters" that issued Monday's ruling was the same body that last year banned the Muslim Brotherhood, the Islamist movement that propelled Mursi to the presidency in a 2012 election.
Western governments have expressed concern about the Egyptian government's repressive policies since he was ousted but there has been little sign of meaningful pressure.
In an official document released after the ruling, the court did not comment on the specific accusations made against April 6 by the lawyer, Ashraf Saeed. But it said the issues raised represented a "real danger".
"It is necessary to immunise the country from this danger," it said.
The lawyer had also accused April 6 of seeking protection from the United States, using media to cause "anarchy" and attacking security institutions - a reference to the storming of the headquarters of Egypt's domestic spying agency in 2011.
In December, three leading members of April 6 - Ahmed Maher, Mohamed Adel and Ahmed Douma - were sentenced to three years in prison on charges including protesting illegally. Their appeals were rejected.
"We will continue our activities and we will state our opinion as we wish," April 6 said in a statement.
In a separate ruling on Monday, a court handed down a death sentence on the leader of the Muslim Brotherhood and 682 supporters.
Abdel Fattah al-Sisi, the general who toppled Mursi, is expected to easily win the presidential election to be held on May 26-27. (Writing by Tom Perry; Editing by Michael Georgy and Angus MacSwan)