ALGIERS (Reuters) - Thousands of people in cities across Algeria marched on Friday to protest against President Abdelaziz Bouteflika’s plan to seek a fifth term and police used tear gas to disperse crowds in the capital.

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The 81-year-old Bouteflika, in office since 1999, will contest the April 18 presidential election despite concerns over his health. He has been seen in public only a handful of times since suffering a stroke in 2013.

“No to Bouteflika and no to Said,” a crowd chanted while marching through the center of Algiers. The president’s youngest brother Said Bouteflika is a presidential adviser.

Reuters journalists filmed riot police firing tear gas over a crowd that ran to escape.

Publicized on social media, the demonstrations went ahead despite mosque preachers warning of possible violence.

“We and the security (forces) are brothers,” some protesters called in the capital.

Protests were held in at least five other cities, including Oran, Tizi Ouzou, Bejaia, Annaba and Setif, news website TSA said, citing witnesses and posting videos.

Bouteflika’s re-election bid comes after the ruling FLN party picked him as its official presidential candidate. Several political parties, trade unions and business organizations have already said they would back him, and he is expected to win easily as the opposition remains weak and divided.

Strikes and protests over social and economic grievances are frequent in Algeria, but are generally localized and do not touch on national politics.

More than a quarter of Algerians under 30 are unemployed, according to official figures, and many feel disconnected from a ruling elite made up of veteran fighters from Algeria’s 1954-1962 independence war with France.

While the “Arab Spring” protests of 2011 toppled leaders in fellow North African countries Tunisia and Egypt, Algerian security forces managed to contain those mass demonstrations.

The government was able to further calm tensions by increasing spending as oil and gas revenues, the country’s lifeline, flowed freely from crude sold at $100 a barrel.

Bouteflika’s office said late on Thursday that he would travel to Switzerland on Sunday for “routine medical checks”, without giving further details. He has traveled before to Geneva for medical tests.

Bouteflika remains popular with many Algerians, who credit him with ending a long civil war by offering an amnesty to former Islamist militant fighters.

His re-election would provide short-term stability for the FLN, the army and business tycoons, and postpone a potentially difficult succession.

Algeria is a key gas supplier to Europe and an ally of the United States in the fight against Islamist militants in the Sahel region of North Africa.