Egyptian authorities on Sunday banned Yemeni rights activist and Nobel Peace Prize winner Tawakkol Karman from entering the country for "security reasons," airport officials told AFP.
Karman was held at Cairo airport on arrival and ordered to return on the flight back to Yemen, the officials said without providing further details.
The first Arab woman to win the Nobel peace prize has voiced support for loyalists of deposed Islamist president Mohamed Morsi and described his ouster by the military in July as undemocratic.
Morsi supporters said in a statement that Karman had been due to make an appearance at a Cairo sit-in where backers of the deposed president have rallied for more than a month.
The Anti-Coup Alliance said Vice President Mohamed ElBaradei, also a Nobel laureate, "is to be held responsible for banning activists and Nobel Prize winners from entering Egypt."
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They condemned the move as a violation of human rights and freedom of speech, which they say is evidence of the "police/military state" in Egypt.
Morsi's July 3 removal from office has intensified divisions in the Arab world's most populous nation.
His supporters have vowed to keep fighting for his reinstatement with two large sit-ins that have brought parts of the capital to a halt, one of them at Rabaa al-Adawiya where they said Karman was due to make an appearance.
Karman was a leading figure during the 2011 youth uprising in Yemen that eventually forced out veteran president Ali Abdullah Saleh, is the first Arab woman to win the Nobel peace prize.
Egypt's interim leaders have said there is no turning back from the army-drafted roadmap that foresees new elections in 2014.
Intense diplomatic efforts are currently underway to find a peaceful solution to the political crisis.