* Activists rounded up over Moscow protest
* Opposition wants its candidates in local vote
* Government says they are barred due to lack of signatures
* Protests intended to turn heat up on Putin
Russian police arrested more than 500 people on Saturday, including prominent activists, around a political protest in Moscow to demand that members of the opposition be allowed to run in a local election later this year.
The protest, which authorities declared illegal beforehand, did not represent a major challenge to President Vladimir Putin and his allies who have the resources to break up such demonstrations and jail people.
With several thousand people present, however, it showed how many activists and especially younger people are unafraid to challenge authorities on the streets and intend to keep pressing for the Kremlin to open up Russia's tightly-choreographed political system to competition.
Chants of "Russia without Putin" and "Putin resign" echoed through central Moscow as guardsmen clad in riot gear beat back protesters with batons and roughly detained people.
At least one woman appeared to suffered serious head wounds.
Jailed opposition leader Alexei Navalny had called for the protest, near the Moscow mayor's office, to persuade authorities to allow opposition-minded candidates to run in a Sept. 8 vote in Moscow, which they have been barred from.
The opposition has no seats in parliament and is starved of air time on state TV where many Russians still get their news.
Opinion polls in the past have shown support for Navalny, a lawyer and anti-corruption activist, only in the single digits. But backers note he won almost a third of the vote in a 2013 Moscow mayoral race and say his movement could build momentum in the Russian capital if allowed to compete fairly.
Though Putin's approval rating is still high at well over 60 percent, it is lower than it used to be due to discontent over years of falling incomes. Last year, the 66-year-old former KGB intelligence officer won a landslide re-election and a new six-year term until 2024.
Burnishing his man of action image, Putin spent Saturday diving to the bottom of the Gulf of Finland in a mini-submarine to pay tribute to a Soviet submarine that sunk there during World War Two.
Authorities say the Moscow local election candidates were prohibited from running because they had failed to collect a sufficient number of genuine signatures in their support, an allegation the opposition rejects as false.
OVD-Info, an independent monitoring group, said police had detained at least 520 people before or at Saturday's protest. In the past many of those taken into custody have often been released later on the same day.
Police put participation at more than 3,500 people, of whom it said around 700 people were journalists and bloggers.
Reuters witnesses said some of those detained appeared to be ordinary passersby in the wrong place at the wrong time.
One of those detained, Alexander Latyshev, 45, said he had came from the nearby Vladimir region to discuss business with an associate and been randomly detained. "I was just sitting on a bench (when they took me)," he told Reuters inside a police bus.
Before the protest, police detained activists to prevent them from attending the protest and blocked off some streets.
During the demonstration, they raided an office being used by Navalny's supporters to live-stream the protest.
Under Russian law, the location and timing of such protests needs to be agreed with authorities beforehand, something that was not done in this case.
Kremlin critic Navalny was jailed for 30 days on Wednesday and other members of the opposition have had their homes searched. Ilya Yashin, a Navalny ally, said on Facebook on Saturday that police had searched his Moscow flat overnight before detaining him and driving him out of the Russian capital.
Kira Yarmysh, Navalny's spokeswoman, said on Twitter she and another activist had been detained on Saturday morning. Other prominent activists, Dmitry Gudkov and Lyubov Sobol, were detained later.
Moscow Mayor Sergei Sobyanin, a Putin ally, had warned beforehand that authorities would act decisively against the risk of "serious provocations".
"Order in the city will be maintained," he said.
Russia's Investigative Committee, the police's investigative arm, has already opened a criminal investigation into an opposition rally in June which it said may have obstructed the work of Moscow's electoral commission.
An authorised protest in Moscow last weekend calling for the disbarred candidates to be registered was attended by more than 20,000 people, according to the White Counter monitoring group.