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ALMATY/NUR SULTAN (Reuters) - Police in Kazakhstan detained dozens of opposition supporters on Sunday who took to the streets after the death of an anti-government activist in a detention center.

Activist Dulat Agadil died in a jail cell on Feb. 25, a day after police took him in on charges of contempt of court and insulting a judge. The authorities - including President Kassym-Jomart Tokayev - have said his death was caused by acute cardiovascular failure, ruling out any foul play.

Agadil’s case has galvanized the oil-rich Central Asian nations’ opposition movement, triggering several small-scale protests during the week and calls for a larger one on Sunday that would bring together groups that otherwise rarely coordinate their activities.

But police in the capital, Nur-Sultan, and the biggest city, Almaty, detained dozens of activists while they were still walking toward the rallying points. In Almaty, policemen including some in riot gear cordoned off the city’s main square and Reuters reporters saw them detain some 40 people.

A few people shouted “Wake up Kazakhstan!” or “Old man, go away!” - an opposition slogan aimed at Tokayev’s predecessor and patron Nursultan Nazarbayev who retains sweeping powers even after giving up the presidency a year ago.

In Nur-Sultan, some protesters carried posters that read “Dulat is a victim of the system” and “No to dictatorship”. A Reuters reporter saw police detain about 30 people there.

Street protests are illegal in the former Soviet republic of 19 million unless sanctioned by the authorities.