Algerian President Abdelaziz Bouteflika has warned of a risk of "chaos" amid mass protests against his plans for re-election.
In a letter published by Algeria's official APS news agency, 82-year-old Mr Bouteflika praised demonstrators for "peacefully expressing their opinions".
But he also urged "vigilance" against possible infiltration of the protests by "domestic and foreign" forces.
Mr Bouteflika announced last month that he would seek a fifth term.
He has ruled Algeria for 20 years but has never been seen in public since he had a stroke in 2013.