The anti-capitalist Occupy movement says several of its supporters have chained themselves to the pulpit of St. Paul's Cathedral.
The action marks one year since the movement's protesters began their camp outside the cathedral.
The St Paul's camp was forcibly removed by authorities back in February this year.
City of London Police are at the cathedral but the women from the anti-corporate group are understood still to be inside.
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Cloud and outbreaks of rain will ease away eastwards during the morning, clearing the east coast around midday with sunny spells developing.
Police and emergency services are attending the scene.
Rebel-held east remains at risk while the pumping station supplying the western part of the city is once again in operation.