MOSCOW — Mass rallies in Saint Petersburg and Moscow were taking place on Wednesday evening even as Russian authorities detained hundreds of supporters of Kremlin critic Alexei Navalny.

Many were seized before protests even began, including two top Navalny associates in the Russian capital. Allies of the imprisoned Russian opposition leader had called for rallies in around 100 cities across Russia on Wednesday.

The demonstrations were arranged to coincide on the same day as Russian President Vladimir Putin's annual address. The protests in Russia's two largest cities were held under heavy police presence as UN experts said Navalny should be evacuated for urgent medical treatment abroad.

Navalny's team said on Twitter the Moscow itinerary would include Tverskaya Street, which is the main road leading up to the Kremlin, the State Duma and the Bolshoi Theater, all the way to the FSB's headquarters.

Footage released by news agencies showed thousands of protesters marching through central Moscow. Protests in support of Navalny began in each city around 7 p.m. local time and moved west across the sprawling country.

In Vladivostok, several hundred protesters marched peacefully under police surveillance, chanting "freedom for political prisoners".

According to NGOs, at least 200 people were detained in at least 40 different cities, including Moscow and St Petersburg.

In the capital city, police had ramped up security presence on the streets and closed Red Square on Wednesday. The country's Interior Ministry and the Prosecutor's Office had warned days earlier that they would take "measures" if unauthorized rallies were held.

Previous nationwide protests in support of Navalny in January resulted in more than 11,000 arrests and at least seven lengthy prison sentences on charges of "violence" against the police.

Two close associates of the Kremlin critic were also detained hours before Wednesday's protests were due to begin.

Lyubov Sobol, a close ally of Navalny, was removed from a taxi near a Moscow metro station on Wednesday morning by people in "uniform", her lawyer said on Twitter.

Meanwhile, police reportedly arrested Navalny's spokeswoman Kira Yarmysh near the entrance of her apartment building, according to her legal team. Yarmysh has been under house arrest on charges related to January protests in support of the politician.

"The detention of supporters of Alexei Navalny in advance of planned protests in Russia today are deplorable," said Charles Michel, president of the European Council. "Authorities must respect the right to assembly," he added on Twitter.

"I urge that the necessary and quality medical care be granted to Alexei Navalny and that he be released from prison."

Navalny was arrested in January upon his return from Germany, where he had spent five months recovering from a nerve agent poisoning he blames on the Kremlin. Russian officials have rejected the accusation.

He was sentenced to a 2 and 1/2 years prison sentence for a 2014 fraud charge, which human rights organizations have labeled "political".

On March 31, Navalny began a three-week hunger strike in prison to protest against officials' refusal to let him be treated by his doctor for back pain and other ailments. Russia's penitentiary service has said Navalny was getting all the medical help he needs.

But on Saturday, Navalny's personal physician said the opposition leader's health was deteriorating and has demanded that he be transferred to a civilian hospital. — Euronews