Image caption Moscow campaign: Most of Mr Navalny's supporters are young, aspirational voters

Russian police have broken into a Moscow apartment and seized campaign materials allegedly produced illegally for opposition leader Alexei Navalny.

The police arrested four people there, said to be pro-Navalny activists. The raid followed a tip-off from a rival candidate in the Moscow mayoral race.

Mr Navalny, a fierce critic of President Vladimir Putin, wants to be elected mayor in the city next month.

One of his aides said the apartment was not being used by his campaign team.

"It is just the private apartment of some activists. It is not linked to Navalny," his spokeswoman Anna Veduta said.

Police found about two tonnes of campaign publications in the flat - more than officially declared by Mr Navalny, a source at the Moscow Central Electoral Commission told RIA Novosti news agency.

Russian electoral law says a candidate must state on all campaign leaflets and posters that they were paid for out of his or her official electoral fund. The total print run also has to be stated on the campaign materials, the Russian website Lenta.ru reports.

Rival's tip-off

Lenta.ru carries a video apparently showing the police raid, in which a man is seen being dragged out of the apartment and bundled downstairs.

According to Russian interior ministry officials, police broke in after the occupants refused to answer the door and smoke appeared from one of the windows.

The tip-off about allegedly "illegal" production of campaign materials for Mr Navalny came from Nikolai Levichev, the mayoral candidate for the leftist A Just Russia party.

A Just Russia sees itself as a socialist alternative to the dominant pro-Putin groups, and has 64 of the 450 seats in Russia's lower house, the State Duma.

Opinion polls suggest that neither Mr Levichev nor Mr Navalny will be able to unseat the pro-Putin incumbent mayor, Sergei Sobyanin.

Mr Sobyanin has refused to take part in any radio or TV debates during the campaign for the 8 September election.

He was appointed Moscow mayor in October 2010, after the dismissal of Yuri Luzhkov, who was accused of corruption and mismanagement. Mr Sobyanin was previously Mr Putin's chief-of-staff and, before that, governor of the oil-rich Tyumen region in Siberia.

On Monday Russian prosecutors said they were investigating Mr Navalny's campaign funding, after Kremlin supporters alleged that he was getting illegal donations from foreigners.

Mr Navalny, who is appealing against a jail sentence, denied the funding claims. He stressed that donations to his campaign were all checked by the authorities.

Last month Mr Navalny, 37, was convicted and given a five-year jail term for embezzlement, though he was freed from custody pending an appeal.

The trial and jail term have been criticised by the US, EU and human rights groups as politically motivated.