(AFP) - The hand-picked successor of Kazakhstan's longtime ruler has won a sweeping victory in a presidential election, authorities said on Monday, after a voting day marred by protests and arrests.

Kassym-Jomart Tokayev took 70.8 percent of the vote in Sunday's election, the Central Election Commission said, with his nearest rival, opposition candidate Amirzhan Kosanov, taking 16.2 percent.

Tokayev's victory was never in doubt after he received the blessing of former president Nursultan Nazarbayev, who led the Central Asian nation for three decades before his shock resignation in March.

But the vote Sunday was marked by the biggest protests the Muslim-majority country has seen in at least three years, as demonstrators urged a boycott of what they said was a fixed election.

The interior ministry said about 500 people were arrested on Sunday, with deputy minister Marat Kozhayev blaming "radical elements" for holding "unsanctioned" rallies.

Activists denounced the arrests and there were claims of ballot stuffing, but Tokayev insisted the election had been "a competition".

"This shows that our level of political culture has seriously increased. There is much work ahead," he said in comments broadcast Monday on state television.

Authorities said they would investigate all allegations of vote violations.

No vote in Kazakhstan has ever been recognised as fully democratic by the Organisation for Security and Co-operation in Europe (OSCE), which sent more than 300 observers to monitor this election.

The OSCE mission is expected to provide its verdict on the polls at 0900 GMT on Monday.

Tokayev's opponents were quick to congratulate him on victory, including second-place finisher Kosanov, a journalist who positioned himself as the ballot's sole opposition candidate.

"This political transition will take place with the participation of the people," Kosanov said, criticising "foreign-based" opposition figures who had encouraged voters to boycott the poll.

"People are ready for an equal dialogue with authorities," he said.

Kosanov had previously said that if the count was fair the result would go to a second round.

- Snap trials, more arrests -

Toleutay Rakhimbekov, a career government official who ran on a rural issues campaign for the Aul (Village) party, said the result was "proof of the population of Kazakhstan's trust and confidence in (Tokayev's) rule."

Third place was claimed by Daniya Yespayeva, the first woman candidate in a Kazakh presidential election, who scored 5.2 percent of the ballot according to the CEC.

Sunday's protests took place in the capital Nur-Sultan, renamed after Nazarbayev following his resignation, and largest city Almaty.

Several hundred rallied in Almaty before police moved in to detain protesters.

Journalists, including two from AFP and reporters for Radio Free Europe/Radio Liberty, were among those detained in Almaty but were later released.

About 50 more people were detained on Monday as they attempted to gather again in the central Almaty square where Sunday's protests took place.

Marzhan Aspandiyarova, a civic activist who visited police stations in Almaty on Sunday and Monday, said that snap trials were underway to convict and sentence the protesters.

"Relatives are trying to get more information but it is difficult because the detained had their telephones confiscated."

Nazarbayev's announcement in March that he was stepping down from the presidency and naming Tokayev interim leader shocked Kazakhs who had lived under his rule since Soviet times.

But the 78-year-old, who turned the country of 18 million people into an energy powerhouse while governing with little tolerance for opposition, is still expected to call the shots from behind the scenes.

© Agence France-Presse