By Andrew Osborn and Gleb Stolyarov
MOSCOW, July 27 (Reuters) - Russian police have detained prominent members of the opposition in Moscow ahead of a planned protest later on Saturday which the authorities have declared illegal and warned could become violent.
The demonstration, due to be held outside the Moscow mayor's office from 1100 GMT, is designed to protest against what the opposition says is the unfair exclusion of a raft of opposition-minded candidates from a Sept. 8 local election in Moscow.
The authorities say the candidates were barred from running because they had failed to collect a sufficient number of genuine signatures in their support, an allegation the opposition rejects as false.
Although the election to select members of Moscow city's parliament is not a national one, opposition activists view it as an opportunity to try to gain a foothold in the Russian capital where Kremlin-backed candidates have proved less popular in the past than in other parts of the country.
Kremlin critic Alexei Navalny was jailed for 30 days on Wednesday ahead of Saturday's protest and other members of the opposition have previously 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 activists had their homes searched.
Moscow Mayor Sergei Sobyanin warned on social media that the authorities would act decisively to guarantee public order.
"According to information from law enforcement authorities serious provocations are being prepared which pose a threat to the safety, lives and health of people," Sobyanin said in a statement.
"Attempts at ultimatums, disorder will not lead to anything good. Order in the city will be maintained."
Reuters reporters said there was a heavy police presence in central Moscow ahead of the protest with groups of ununiformed athletic-looking young men accompanying police.
Russia's Investigative Committee 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.
A protest in Moscow last weekend calling for the disbarred candidates to be registered was attended by more than 20,000 people, according to a monitor. (Reporting by Andrew Osborn; Editing by Mark Potter)