The Archbishop of Canterbury Most Rev Justin Welby described the incident by the Houses of Parliament as "brutal and senseless", while Cardinal Vincent Nichols said it was "terrible".
In a tweet, Archbishop Welby said:
Praying tonight for those grieving loved ones - and those injured and traumatised - after the brutal and senseless attack in #Westminster.
— Justin Welby ن (@JustinWelby) March 22, 2017
Meanwhile, MPs gathered in the House of Commons on Thursday for one minute of silence at 9.33am in respect of the victims of yesterday's attack.
Bishop of Leeds Rt Rev Nick Baines is the duty bishop in the House of Lords this week and was in Parliament at the time of the attack.
He told Premier: "London has to carry on. Life will carry on today. I'll be leading prayers at 11am in the House of Lords.
"I think people are too quick to rush to judgement on the whys and wherefores of it. We need a sober reflection on what has happened.
"It's a tragedy. It's a tragedy for the families of those who have been killed and those who've been injured; their lives will change for forever. It provides a challenge to how we respond to it."
Archbishop Justin also retweeted a prayer from the Church of England:
A Prayer for Westminster #Parliament #London #Westinster pic.twitter.com/Ve0Rza486p
— Church of England (@c_of_e) March 22, 2017
Four people have now died - including the attacker - and 29 are injured after a man drove into pedestrians on Westminster Bridge before fatally stabbing a police officer outside the Palace of Westminster.
PC Keith Palmer (pictured below) was unarmed at the time of the attack.
Two members of the public - a woman in her mid 40s and a man in his mid 50s were also killed. Seven of the injured are in critical condition.
Cardinal Nichols tweeted:
We pray tonight for the victims, their families, for all affected by this terrible attack and those who responded so bravely. #Westminster
— Cardinal Nichols (@CardinalNichols) March 22, 2017
Bishop Angaelos, the General Bishop of the Coptic Orthodox Church in Britain, was inside the Houses of Parliament at the time. In the aftermath of the attack, he told his Twitter followers he was safe and well inside.
He went on to thank emergency services for their help:
Thankful for @UKParliament staff and @MetPoliceUK officers who looked after us all so well throughout today's #WestminsterAttack aftermath.
— Bishop Angaelos ن (@BishopAngaelos) March 22, 2017
Westminster Abbey provided hospitality to Parliament staff and members of the emergency services.
Dean of Westminster John Hall tweeted:
Glad to offer hospitality in @wabbey to members and staff and others from #Parliament - sad incident today pic.twitter.com/9IN4vl47ZH
— John Hall (@deanwestminster) March 22, 2017
A spokesperson from the Abbey told Premier they "simply responded to a request for help from a neighbour".
On Thursday, Pope Francis sent a telegram expressing his condolences to Cardinal Vincent Nichols.
It read:
"Deeply saddened to learn of the loss of life and of the injuries caused by the attack in central London, His Holiness Pope Francis expresses his prayerful solidarity with all those affected by this tragedy. Commending those who have died to the loving mercy of Almighty God, His Holiness invokes divine strength and peace upon their grieving families, and he assures the nation of his prayers at this time."
Last night seven people were arrested in six counter-terrorism raids in London, Birmingham and else where according to Mark Rowley, acting deputy commissioner at the Metropolitan Police.
Defence Secretary Sir Michael Fallon told BBC Radio 4's Today programme the "working assumption" was that the attack was linked to "Islamic terrorism in some form".
A vigil will be held at Trafalgar Square at 6.00pm tonight in memory of the victims.
In a statement, the Baptist Church in Great Britain said its "thoughts and prayers are with all those caught up in the terror incident at Westminster" while the Methodist Church in Great Britain urged believers in join them in praying for the situation.
The President and Vice-President of the Methodist Conference said: "We ask all Methodists to join us in prayer as we unite with others across the UK in the wake of today's attack at Westminster.
"We pray for those who were attacked and for their families, for the emergency services who responded so quickly and for the whole country at this terrible time."
Listen to Premier's Alex Williams speaking with Bishop of Croydon Rt Rev Nick Baines: