* Focus is on Moscow, where big protests erupted this summer
* Putin defends plurality of vote
* Opposition's Navalny advised tactical voting (Adds context and detail after polls close in Moscow)
By Gleb Stolyarov and Andrew Osborn
MOSCOW, Sept 8 (Reuters) - Residents of Moscow voted on Sunday in one of the most closely watched local elections in years after the exclusion of many opposition candidates triggered the biggest protests in the Russian capital for nearly a decade.
Protests erupted in mid-July after the Central Election Commission refused to register a large numbers of opposition-minded candidates, saying they had failed to collect enough signatures from genuine backers - a response that President Vladimir Putin endorsed on Sunday after casting his ballot.
Those excluded, including allies of prominent opposition politician Alexei Navalny, denounced the move as a ruse designed to stop them winning seats in Moscow's parliament.
Local or regional elections took place across all of Russia's 11 time zones on Sunday. But the main focus was on Moscow after this summer's demonstrations there turned into the biggest sustained protest movement in Russia since 2011-2013.
Data from the election commission suggested the turnout in Moscow would be a little more than 20%. Several videos shot in polling stations showed some voters openly stuffing ballot boxes with multiple voting slips circulated on social media.
Though local, the Moscow election was earmarked by Navalny and his allies as an opportunity to make inroads against the ruling pro-Putin United Russia party ahead of a national parliamentary election in 2021.
The party's popularity is at its lowest level in more than a decade.
Putin, asked after voting in central Moscow if he would have preferred more diversity and a larger number of election candidates, told reporters: "In some countries there are 30, 50 and 100 (candidates). It's not important how many there are, but of what quality they are."
At more than 60%, Putin's own popularity rating is much higher than most Western leaders, though lower than it has been previously. The former KGB officer won a landslide election victory last year that will keep him in office until 2024.
United Russia's popularity is suffering from discontent over a move to raise the retirement age at a time of steadily falling incomes and its Moscow candidates rebranded as independents in an apparent effort to distance themselves from it formally.
Navalny advised his supporters to vote tactically across Russia to reduce the party's influence.
One 25-year-old Muscovite, a lawyer who gave his name only as Vladislav, said he had voted tactically for Sergey Mitrokhin, of the opposition Yabloko party, "because I'm tired of United Russia, stealing and everything that follows".
"I hope that smart voting will work ... We have to start changing something (and) ... there is nothing to lose any more," he told Reuters.
Police detained about a dozen opposition activists near Moscow's city hall on Sunday. The activists were wearing T-shirts drawing attention to the fate of people the authorities have charged in connection with this summer's protests. (Additional reporting by Andrey Kuzmin Writing by Andrew Osborn Editing by David Goodman)