By Polina Nikolskaya and Andrew Osborn
VLADIVOSTOK, Russia/MOSCOW (Reuters) - Russia's ruling United Russia Party suffered a rare setback in regional elections despite winning most of the seats, a reversal its leaders and election chiefs blamed on unpopular plans to raise the pension age.
The results in weekend voting for heads of about one third of Russia's regions were the worst for United Russia, which backs President Vladimir Putin, since elections for regional leaders were re-introduced in 2012.
Four Kremlin-backed candidates were forced into run-off votes. Two were beaten into second place -- by a communist and a nationalist -- and two finished first but failed to win the more than 50 percent of votes needed for outright victory.
Opponents of the pension reforms, which envisage raising the retirement age for men to 65 from 60 and to 60 from 55 for women, staged protests across Russia as voting took place.
Police detained just over 1,000 people, OVD-Info, a rights organisation that monitors detentions, said. Jailed opposition leader Alexei Navalny had called for the protests.
Ella Pamfilova, the head of the Central Election Commission, said it was obvious the planned pension changes had prompted voters to register their discontent at the ballot box, something she said was a sign of genuine political competition.
"It's a good lesson for everyone," she told a news conference. "It's very useful for the party of power to get a bit of a jolt."
Speaking in the far eastern city of Vladivostok, Putin told government officials he was unfazed by the fact that re-runs would be needed in four regions.
"It's an absolutely normal phenomenon," he said.
The region the includes Vladivistok is one of those where a second vote will be required. The man Putin appointed acting governor there last year failed to pass the 50-percent threshold on Sunday after a communist won almost a quarter of the votes.
"You don't need to hunt for complex explanations as to why this happened," said Ivan, a voter in Vladivostok who declined to give his surname. "Everything can be explained by the fall in the authorities' popularity because of pension reform."
United Russia also lost ground to the Communist Party and the nationalist Liberal Democratic Party of Russia (LDPR) in some areas in weekend elections to regional parliaments.
The proposed pension changes, which are being considered by the national parliament, have shaved about 15 percentage points off Putin's popularity rating, opinion polls show, although his ratings remain high.
They are the most unpopular government measure since a 2005 move to scrap Soviet-era benefits, though Putin amended the planned reforms after initial protests. Average life expectancy for men in Russia is 66 and for women 77.
Prime Minister Dmitry Medvedev, the leader of United Russia and a Putin ally, told party activists on Sunday night he deemed the results "worthy" given the election campaign had taken place in what he called difficult conditions.
"... There's a heated public discussion in society right now about a whole raft of changes, including changes to pension law. That undoubtedly ratcheted up the intensity of the campaign and of the political battle," Medvedev said.
In Moscow, Putin's former chief-of-staff Sergei Sobyanin was re-elected mayor with about 70 percent of the votes. But despite a huge get-out-the-vote campaign, turnout was just 30 percent as many Muscovites stayed away. That was less than the turnout at the previous mayoral election in 2013.
Putin was in Vladivostok on Monday to meet Asian leaders at an economic forum.
Vladivostok resident Alexei, 41, who works in the local ship building industry, said he had voted for the communist candidate as a protest vote and planned to do the same in the run-off.
"We really don't like United Russia here and Putin and his comrades at the moment," he said.
United Russia remained the dominant force in the national parliament in the last election in 2016. The next is due in 2021.
(Writing by Andrew Osborn; Editing by Timothy Heritage)