Egypt barred the Yemeni Nobel peace laureate Tawakul Karman from entering the country on Sunday and put her on a flight back to Dubai, security sources said.
State news agency Mena said Karman, who had previously announced her solidarity with supporters of Mohamed Morsi, overthrown by the army a month ago, was on a list of people who were not allowed to enter Egypt.
A spokesman for Morsi's Muslim Brotherhood said Karman had recently joined demonstrations in Cairo demanding the former leader be reinstated.
Karman's Twitter feed on Sunday said the writer and activist had been held at Cairo airport and prevented from joining protests. She was sent back on the same plane she flew in on, the security sources said.
The Brotherhood criticised Karman's deportation and said it was reminiscent of the rule of former autocrat Hosni Mubarak.
"This is an abandonment of the gains of Egypt's January revolution. The government is reproducing the practices of Mubarak's state security," said Yasser Ali, a Brotherhood official and former presidential spokesman.
Karman, a 34-year-old mother of three, who became a figure of symbolic importance in the 2011 Yemeni uprising, became the first Arab woman and second Muslim woman to win the Nobel peace prize. In Yemen she is called the "Iron Woman" and "Mother of the Revolution".
A member of Yemen's leading Islamic opposition party, the Islah, Karman had denounced the army's toppling of Morsi, calling it a coup and a "blow to democracy".
In a statement on Friday, she said it had weakened moderate political Islam and strengthened the hand of religious extremists in the Arab world.
"We can't allow this sense of disappointment in democracy to grow. This is terrifying. Rest assured the first beneficiary of the weakening currents of political Islam are violent terror groups."