Tens of thousands of people are marching in the Algerian capital, Algiers, in the latest protest against President Abdelaziz Bouteflika.
The demonstrations - now in their third week - were triggered by the ailing president's decision to seek re-election for a fifth term in April.
Mr Bouteflika has ruled Algeria for 20 years but has rarely been seen in public since he had a stroke in 2013.
He has warned that the protests could plunge the country into "chaos".
In a letter published by Algeria's official APS news agency on Thursday, the 82-year-old leader urged "vigilance" against "domestic and foreign" forces that might infiltrate the demonstrations.
But he also praised the demonstrators for "peacefully expressing their opinions".
The president who doesn't speak
Algeria protests: The beginning of the end?