More than 1,700 people have now been detained by police at rallies in support of jailed Kremlin critic Alexei Navalny in dozens of cities across Russia, an independent monitor has reported.

Thousands of people took to the streets last evening to demand freedom and proper medical attention for Navalny, who has been hunger strike for three weeks in a penal colony outside Moscow.

The opposition staged demonstrations in dozens of Russian cities, with the largest rallies taking place in Moscow.

The OVD-Info monitoring group, which tracks detentions at opposition protests, said that by this morning police had detained "more than 1,783 people in 97 cities".

Yesterday morning it was reported just over 1,000 people had been detained.

In Russia, participation in unauthorised demonstrations can lead to a fine or several days in jail.

The majority of detentions - 805 - took place in Saint Petersburg, where police violently dispersed crowds with shock sticks.

Navalny's press secretary Kira Yarmysh was among those detained. She was given a ten-day stint behind bars for calling on people to join unauthorised rallies.

Alexei Navalny has been on hunger strike for three weeks in a penal colony outside Moscow

Key Navalny aide Lyubov Sobol was also taken into police custody ahead of the rally in Moscow and is due to appear in court later today.

Yesterday's rallies did not match in size those held in the winter when Navalny was arrested after returning to Russia from Germany.

Tens of thousands took to the streets despite freezing temperatures while more than 11,000 people were detained.

Navalny, 44, was arrested when he returned to Russia in January after months recovering in Germany from a near-fatal nerve agent poisoning he blames on the Kremlin - an accusation it rejects.

He was sentenced to two-and-a-half years over an old fraud conviction and has been serving time in a penal colony about 100 kilometres east of Moscow.

His health has been failing since he launched his hunger strike to demand proper medical care for a range of ailments, including back pain and numbness in his limbs.