ALGIERS — Tens of thousands of Algerians defied large contingents of riot police on Friday and resumed mass demonstrations against President Abdelaziz Bouteflika posing the biggest challenge to his 20-year-old rule.

Unusually, one of the most popular imams, or prayer leaders, in Algiers did not pray for the president as he does every Friday, and only wished the best for Algeria and its people.

Several lawmakers of the ruling FLN party have resigned to join the mass anti-government protests, the private Ashourouq TV station said. No details were immediately available.

Train and metro services in Algiers were suspended without explanation before Algerians gathered once again in several cities to press the 82-year-old Bouteflika to step down.

The ailing Bouteflika is in hospital in Geneva and has rarely been seen in public since suffering a stroke in 2013. On Thursday he issued his first warning to protesters, saying the unrest, now entering its third week, could destabilize the country.

Swiss police detained Algerian opposition candidate Rachid Nekkaz for trespassing by entering the hospital where Bouteflika is being treated.

He had traveled there to demand to see himself whether the president is fit for office.

“There are 40 million Algerians who want to know, where is the Algerian president, Mr. Abdulaziz Bouteflika,” he told reporters before entering the facility.

The demonstrations, the largest since the 2011 “Arab Spring”, pose the most substantial threat to the president, who is standing for re-election on April 18.

Many Algerians, tired of the dominance of elderly veterans of the 1954-1962 war of independence against France, are demanding Bouteflika step down, but despite ill health, he has submitted his candidacy papers.

Bouteflika has offered to limit his term after the election — and even to change the “system” that runs the country —but people from different classes of society, including students and young families, are still on the street.

Some long-time allies of Bouteflika, including members of the ruling party, have expressed support for the protesters, revealing cracks within a ruling elite long regarded as invincible.

Older Algerians with dark memories of a civil war in the 1990s have tolerated crackdowns on dissent in exchange for stability, and Bouteflika avoided the kind of uprising that toppled Arab leaders in 2011 because the state had enough foreign reserves to boost state spending.

But the young want jobs and are tired of what they perceive as widespread corruption. More than a quarter of Algerians under 30 are unemployed.

Algerians have been urged to stage a “March of 20 Million” on Friday by an anonymous party on social media. — Reuters