Police in Kazakhstan have arrested dozens of people in the Central Asian country's largest city after an activist's death in jail triggered diplomatic condemnation and calls for anti-government rallies.

In the capital Almaty, policemen cordoned off the city's main square and about 40 people were detained.

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Elsewhere in the city, 26 members of Oyan Qazaqstan, one of the groups that called for a rally, were detained on Sunday before they had a chance to reach the protest site.

"According to our information, they have not been released yet," Dimash Alzhanov, one of Oyan Qazaqstan's founding members, told AFP news agency by telephone.

One man - who AFP correspondents saw bundled into a van full of detainees by black-clad police - appealed to "the lawmakers in the European parliament", which sent a delegation to Kazakhstan last month.

"My constitutional rights are being violated. This is the 21st century," the man cried out as he was shoved into the vehicle.

A few people shouted "Wake up Kazakhstan" and "Old man, go away" - an opposition slogan aimed at President Kassym-Jomart Tokayev's predecessor and patron, Nursultan Nazarbayev, who retains sweeping powers even after giving up the presidency a year ago.

Two journalists were also among those detained on Sunday.

Beaten in detention

Activist Dulat Agadil died in a jail cell on February 25, a day after police took him in on charges of contempt of court and insulting a judge.

People attend a meeting in memory of Kazakh anti-government activist Dulat Agadil [Pavel Mikheyev/Reuters]

The United States and the United Kingdom raised concern over his death this week and called for a "thorough" investigation.

The authorities - including President Tokayev - have said his death was caused by acute cardiovascular failure, ruling out any foul play.

Activists have cited video footage of Agadil's corpse with bruises as evidence he was beaten in detention, rather than dying of heart failure, as police said on Tuesday.

The state prosecutor's office on Friday called on citizens not to make "hasty conclusions" about the bruises, which the office said was common on corpses.

Kazakhstan regularly cracks down on citizens who attempt to hold rallies as street protests are illegal in the former Soviet republic unless sanctioned by the authorities.

The country is in the process of changing its legislation on public assemblies, and a draft of the new law has been released for public discussion.

But civil society groups in the Central Asian state have expressed dissatisfaction with the draft law, which they argue would introduce additional restrictions.