Thousands of people across Russia have protested against a deeply unpopular pension reform, despite concessions offered recently by President Vladimir Putin.

"Today, we are holding an all-Russia protest against this cannibalistic reform," Communist Party leader Gennady Zyuganov told a crowd of several thousand protesters gathered in Moscow on September 2.

Demonstrations were reported in many other cities, including St. Petersburg, Russia’s second-largest city, Yekaterinburg in the Ural Mountains, Novosibirsk in Siberia, and the Far Eastern port city of Vladivostok.

In July, Russia’s parliament gave preliminary backing to a government plan to raise the retirement age to 63 for women by 2034 and to 65 for men by 2028.

Tens of thousands have rallied against the proposed reform in recent weeks, and Putin's popularity in opinion polls has noticeably decreased.

In response to the outburst of public anger, Putin promised in a televised address on August 29 to soften the controversial measures.

The president proposed raising the state pension age by five years to 60 years for women, instead of the earlier proposed eight-year hike, while a five-year increase for men would stay.

He also proposed allowing women with at least three children to retire earlier.

Currently, the retirement age is 55 for women and 60 for men.

Russian men and women have life expectancies of 66 and 77, respectively, according to the World Health Organization, and critics have warned that many won’t live long enough to claim a pension.

The issue of raising the retirement age has turned into the biggest domestic challenge Putin has faced since winning another term as president in March.

On September 2, around 9,000 people took part in a protest in Moscow’s Sakharov Avenue, according to White Counter, an independent activist group that tracks turnout at demonstrations, while the Interior Ministry put the numbers at 6,000.

Many participants carried the red flags and banners of the main organizer of the rally, the Communist Party.

One of the banners displayed at the rally read, "We do not trust United Russia,” the country’s ruling party.

A separate demonstration in Moscow, organized by the Just Russia party, attracted a further 1,500 people on Suvorovskaya Square, police said.

One activist was reportly detained during the protest.

Elsewhere in Russia, reports said that at least 1,500 demonstrators took to the streets of St. Petersburg.

In the Volga city of Samara, some protesters in a 1,000-strong crowd called for Putin’s resignation, the AFP news agency reported.

Reports said that 2,500 people joined a similar rally in Novosibirsk.

In Vladivostok, the city administration said more than 200 people gathered for a protest, but organizers said they numbered about 2,000.

In Yekaterinburg, around 450 people took part in a protest during which demonstrators held portraits of politicians who have voiced support for pension reform, according to local newspaper Nasha Gazeta.

In the south, rallies took place in the Black Sea port of Novorossiisk, Astrakhan, Rostov-on-Don, and in the capitals of the North Caucasus regions of Karachayevo-Cherkessia and Kabardino-Balkaria, the Interfax news agency reported.

Demonstrators also took to the streets in the capital of Simferopol in Crimea, the Black Sea peninsula annexed by Russia from Ukraine in March 2014.

Opposition politician Aleksei Navalny called for more mass protests against the pension reform on September 9.

Ahead of the planned demonstrations, Navalny was sentenced to 30 days in jail on August 27 for helping to organize an unsanctioned street rally in Moscow in January.

With reporting by AFP, Reuters, and Interfax