Algerian authorities halted train and metro services in the capital Algiers on Friday amid heavy security ahead of expected protests against President Abdelaziz Bouteflika.

Mass demonstrations now entering their third week have posed the biggest challenge to the ailing 82-year-old president, who has ruled for 20 years and is standing for re-election on April18. Tens of thousands of Algerians, tired of the dominance of elderly veterans of the 1954-1962 war of independence against France, have been demanding that Bouteflika step down, but despite ill health, he has submitted his candidacy papers.

Mass protests are expected to start in the afternoon. On Thursday, Bouteflika issued his first warning to protesters, saying the unrest — the biggest since the 2011″Arab Spring” — could destabilise Algeria, a major oil and gas producer.

The president has not spoken in public since suffering a stroke in 2013, and is currently in hospital in Geneva. Algerians have been urged to stage a “March of 20 Million” by an anonymous party on social media, and hundreds of protesters gathered in Algiers ahead of what could be the biggest demonstrations yet in the capital and other cities.

Riot police have been deployed in growing numbers in recent days, but the military has so far stayed in its barracks. “Today will be a decisive day,” said Mohamed, a teacher in Algiers.