CAIRO, May 11 (Reuters) - Egypt's public prosecutor on Saturday ordered the release of a prominent activist detained at Cairo airport after flying from the United States over allegations of inciting a demonstration.

Ahmed Maher was a founder of the April 6 movement that used social media to help kindle the uprising that toppled President Hosni Mubarak in 2011. He is being investigated over a protest outside the interior minister's home in March.

Critics say it is the latest example of Islamist-led authorities cracking down on dissidents since the Muslim Brotherhood's Mohamed Mursi was elected president last year.

The state news agency listed the accusations against Maher as "resisting the authorities, insulting the police, gathering and obstructing traffic". The charge of "gathering" was frequently used in Mubarak's era to crush protests.

The United States, which gives Egypt $1.3 billion in military aid each year, has cited arrest warrants against activists as "evidence of a disturbing trend of growing restrictions on the freedom of expression".

Asked about Maher's arrest on Friday, State Department spokesman Patrick Ventrell said the United States was seeking confirmation but would express concern if it was true.

Ventrell said Maher had been visiting the United States where he had met several senior State Department officials "as do many visiting activists and politicians from across the political spectrum in Egypt".

Another prominent activist, Ahmed Douma, went on trial on Sunday charged with insulting Mursi.

Members of April 6 staged a demonstration late on Friday in front of the prosecutor general's office in Cairo demanding Maher's release.

In March, April 6 led a protest in front of the home of Interior Minister Mohamed Ibrahim demanding the release of activists arrested in a security crackdown. Four of the group's members were arrested after the demonstration. (Reporting by Yasmine Saleh; Editing by Tom Perry and Janet Lawrence)