Russia's opposition leaders have called for the results of Russia's disputed election to be annulled, as popular anger grows over allegations of fraud.

Thousands of people rallied in the capital, Moscow, and in the city of St Petersburg in the last two days, facing off against tens of thousands of police and interior ministry security forces.

Hundreds of protesters were detained in both cities.

Organising themselves on social media websites, opposition groups are spreading the word for a mass rally on Saturday.

The authorities are warning, however, that they will arrest anyone demonstrating without permission.

Al Jazeera's Neave Barker reports from Moscow.