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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 rarely been seen in public since he had a stroke in 2013.

He announced last month that he would stand again for president, prompting large protests in towns and cities across the country, with students, lawyers and journalists taking part. Further large demonstrations are planned for Friday.

Mr Bouteflika later said in another letter that he would step down early if re-elected, but the protests have continued.

The president was transferred to a hospital in Switzerland on 24 February, for what his campaign described as "routine" medical tests. His campaign manager told Algeria's El Khabar newspaper on Thursday that his health raised "no concerns".

A spokesman for the Geneva University Hospitals (HUG), where Mr Bouteflika is staying, said it had received 1,500 phone calls on Tuesday about the president after his location was revealed by French television programme Quotidien, according to French-language media reports.

While most of the calls have reportedly been enquiring about the president's health, a number of videos posted to social media show prank calls from Algerians.