ALGIERS (Reuters) - Algerian President Abdelaziz Bouteflika issued his first warning on Thursday to protesters who have taken to the streets in thousands to demand an end to his 20-year rule, saying the unrest could destabilize the country.

In the latest demonstration, hundreds of lawyers in black robes rallied in downtown Algiers on Thursday as momentum gathered in the country’s most sustained protests since the 2011 Arab Spring.

Bouteflicka, 82 and ailing, has not spoken in public since suffering a stroke in 2013 and he is staying in a hospital in Geneva.

But in a letter reported by the state news agency APS, he said: “Breaking this peaceful expression by any treacherous internal or foreign group may lead to sedition and chaos and resulting crises and woes.”

He did not say who any of these groups might be.

An Islamist insurgency in the 1990s that broke out after the army blocked an election victory by an Islamist party was crushed at the cost of up 200,000 lives. There has been sporadic militant activity in recent years.

The stance taken by the military and security forces will be crucial to how the present situation unfolds.

The military has stayed in barracks throughout the unrest. But analysts and former officials say the generals are likely to intervene if the protests lead to instability in one of Africa’s biggest oil producers.

At the lawyers’ protest, police were deployed to monitor the demonstration but as with previous protests, they did not intervene.

Police attempt to disperse lawyers trying to force their way to the constitutional council during a protest to denounce an offer by President Abdelaziz Bouteflika to run in elections next month but not to serve a full term if re-elected, in Algiers, Algeria March 7, 2019. REUTERS/Zohra Bensemra

Lawyers shouted chanted: “The people want to overthrow the regime” and “Republic, not a kingdom.

Tens of thousands of Algerians, tired of the dominance of elderly veterans of the 1954-1962 war of independence against France, have protested for the past three weeks to urge Bouteflika not to stand in an election scheduled for April 18.

Despite his ill-health, he has submitted his candidacy papers.

The national association of lawyers has demanded that the authorities postpone the election and set up a transitional government.

RULING ELITE UNDER FIRE

The unrest poses the biggest challenge yet to Bouteflika and his inner circle, which includes members of the military, intelligence services and big business figures.

An anonymous call for a general strike has gone largely unheeded but the leadership faces another test - a call for a “March of 20 Million” this Friday.

On Wednesday, the influential Algerian war veterans association expressed support for the so far peaceful protests.

Two branches of powerful Algerian labor union UGTA, representing tens of thousands of workers, also opposed the re-election plan.

Some officials from Bouteflika’s ruling FLN party have turned up at demonstrations, and several public figures have announced their resignations.