Around 5,000 people gathered for a prayer vigil in Trafalgar Square sending a resounding message: #WeAreNotAfraid.
Faith leaders met with police earlier as Londoners insisted they would not be cowed by Wednesday's attack that killed four and injured 50.
Bells at St Martin-in-the-Fields church rang out before silence was observed and candles lit for the dead.
Home Secretary Amber Rudd spoke alongside London Mayor Sadiq Khan and the Metropolitan Police's Acting Commissioner Craig Mackey. 'We have come together to spread a clear message: Londoners will never be cowed by terrorism,' Khan said to cheers from the crowd.
Several Muslims attended alongside Catholic nuns in a strong message as religious leaders spoke of their unity in the face of violent extremism.
The Muslim Council of Britain called for 'solidarity and prayer' and Secretary General Harun Khan joined the thousands in Trafalgar Square.
'This attack was cowardly and depraved,' he said. 'There is no justification for this act whatsoever. The best response to this outrage is to make sure we come together in solidarity and not allow the terrorists to divide us.'
Pope Francis said he was 'deeply saddened' by the attacks and the Archbishop of Canterbury spoke of the 'deep values' that were shown in the response.
'That speaks of – at this time of year as we look forward to Holy Week and Easter – of a God who stands with the suffering, and brings justice, and whose resurrection has given to believer and unbeliever the sense that where we do what is right; where we behave properly; where that generosity and extraordinary sense of duty that leads people to treat a terrorist is shown; where that bravery of someone like PC Keith Palmer is demonstrated, that there is a victory for what is right and good; over what is evil, despairing and bad.
'That was shown yesterday. That is shown not just in our expression of values, but in our practices which define those values. And that is the mood that we must show in the future.'
Ephraim Mirvis, the Chief Rabbi, said: 'Prayers of the Jewish community are with the families of the victims [and] with our brave security services.'