MOSCOW (Reuters) - Thousands protested in central Moscow on Sunday against a proposed increase to the retirement age and the crowd chanted slogans critical of President Vladimir Putin whose approval ratings have been dented by the bill.

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The rally organized by the opposition Libertarian Party chanted “Putin is a thief” and “away with the tsar,” slogans common at anti-Putin and anti-government protests.

The retirement age proposal is politically sensitive for Putin, who was re-elected in March, because it has prompted a series of protests across Russia since it was announced on June 14, the day Russia played the first match of its soccer World Cup.

Around 90 percent of the population oppose the bill, according to a recent opinion poll, and a petition against it has attracted 3 million signatures online.

More than 6,000 people came to Sunday’s rally some 2.4 kilometers (1.5 miles) from the Kremlin, according to White Counter, an NGO that counts participants at rallies using metal detector frames. Police put the number at around 2,500.

People held placards with slogans against the higher retirement age and one read: “stop stealing our future”. Authorities detained two protest organizers, Vladimir Milov, a former deputy energy minister and now an opposition campaigner, told Reuters.

The proposal to raise the retirement age, to 65 from 60 for men and to 63 from 55 for women, is part of an unpopular budget package designed to shore up government finances that is backed by lawmakers.

Putin, who once promised not to raise the retirement age, has tried to distance himself from the pension plan.

This month he said he did not like any of the proposals. He said Russia could avoid raising the retirement age for years, though a decision would have to be made eventually.

“We have to proceed not from emotions, but from the real assessment of economic conditions and prospects of its development and (the development of) the social sphere,” Putin said.

On Saturday, more than 12 thousand rallied on the same street in Moscow, according to the White Counter data.

The changes to the retirement age would be introduced gradually, starting in 2019, Prime Minister Dmitry Medvedev said when presenting the plan. Officials said the measure should help to raise an average pension in Russia, now at around 14,400 rubles ($229.52).