Police across Russia have arrested more than 400 people in connection with demonstrations in support of imprisoned opposition leader Alexei Navalny a human rights group said.

Many were seized before protests even began, including two top Navalny associates in Moscow.

His team called the unsanctioned demonstrations after reports that his health is deteriorating while on hunger strike, which he began March 31.

“The situation with Alexei is indeed critical, and so we moved up the day of the mass protests,” said Vladimir Ashurkov, a Navalny ally and executive director of the Foundation for Fighting Corruption.

“Alexei’s health has sharply deteriorated, and he is in a rather critical condition. Doctors are saying that judging by his test (results), he should be admitted into intensive care.”

Mr Navalny’s organisation called for Moscow protesters to assemble on Manezh Square, just outside the Kremlin walls, but police blocked it off.

Instead, a large crowd gathered at the nearby Russian State Library and another lined Tverskaya Street, a main avenue that leads to the square.

In St Petersburg, police blocked off Palace Square, the vast space outside the Hermitage museum and protesters instead crowded along nearby Nevsky Prospekt.

There were no comprehensive reports of turnout throughout the country and it was unclear if the demonstrations would match the size and intensity of nationwide protests that broke out in January after Mr Navalny, President Vladimir Putin’s most prominent opponent, was arrested.

The OVD-Info group, which tracks political arrests, reported 413 arrests in 60 cities.

Mr Navalny’s team called the nationwide protests for the same day that Mr Putin gave his annual state-of-the-nation address.

In his speech, he denounced foreign governments’ alleged attempts to impose their will on Russia.

Mr Putin, who never publicly uses Mr Navalny’s name, did not specify to whom the denunciation referred, but Western governments have been harshly critical of Mr Navalny’s treatment and have called for his release.

In Moscow, Navalny spokeswoman Kira Yarmysh and Lyubov Sobol, one of his most prominent associates, were detained by police in the morning.

Ms Yarmysh, who was put under house arrest after the January protests, was detained outside her apartment building when she went out during the hour she is allowed to leave, said her lawyer, Veronika Polyakova.

She was taken to a police station and charged with organising an illegal gathering.

Ms Sobol was removed from a taxi by uniformed police, said her lawyer, Vladimir Voronin.

OVD-Info, a group that monitors political arrests and offers aid to detainees, said at least 182 people had been arrested. It also reported that police searched the offices of Mr Navalny’s organisation in Yekaterinbrug and detained a Navalny-affiliated journalist in Khabarovsk.

Navalny, 44, was arrested in January on 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.

Soon after, a court found that his long stay in Germany violated the terms of a suspended sentence he was handed for a 2014 embezzlement conviction and ordered him to serve two and a half years in prison.

He began the hunger strike to protest over prison officials’ refusal to let his doctors visit when he began experiencing severe back pain and a loss of feeling in his legs. The penitentiary service said Mr Navalny was getting all the medical help he needs.

But his physician, Dr Yaroslav Ashikhmin, said recently that test results showed sharply elevated levels of potassium, which can bring on cardiac arrest, and heightened creatinine levels that indicate impaired kidneys and he “could die at any moment”.