Islamic State has claimed responsibility for a series of attacks that began on Friday evening, 129 people are feared dead.
Eight extremists have been killed, including seven in suicide bombings.
200 people are thought to be injured.
A manhunt is under way for accomplices of gunmen as French president Francois Hollande pledged to wage a "merciless" fight against terrorism.
David Cameron will chair a meeting of the Government's Cobra emergency committee on Saturday morning.
Tragic Paris, desperate news of deep tragedy, with heart break for so many. We weep with those affected, pray for deliverance and justice.
— Justin Welby ? (@JustinWelby) November 13, 2015
Pope Francis has condemned the killings in Paris as "mad terrorist violence" and has called for a response to counter the spread of "homicidal hatred".
A spokesman for the Vatican said it condemns the attacks "in the most radical way".
Prince Charles has added his voice to the prayers.
The Prince of Wales is sending a message of support to President Francois Hollande expressing his support for Parisians who have endured a night of devastating terrorist attacks.
A Clarence House spokeswoman said: "Their Royal Highnesses are carrying on with engagements in Albany and Perth today."
Charles turns 67 today.
A state of emergency has been declared after two suicide attacks and a bombing near the Stade de France stadium, shootings at restaurants and a massacre inside a popular music venue in what is the worst violence to hit France since the Second World War.
Officers stormed the Bataclan concert hall where hostages were being held but attackers, wearing suicide belts, blew themselves up, leaving more than 100 people dead.
The national football side was playing a friendly match against Germany at the Stade de France when two suicide attacks and a bombing took place nearby.
President Hollande had been at the stadium but was evacuated.
Tens of thousands of people fell silent at the stadium on Friday night in tribute to those killed and injured.
The first attack is thought to have begun around 10pm French time.
A prayer has been released on Godspace to grieve with those in the French capital:
"Lord we grieve with the people of Paris.
We grieve,
for the lives lost in senseless violence.
Our tears are shed
for those who have been wounded.
Our hearts ache
for those who are consumed by hate.
Lord have mercy,
Christ have mercy,
Lord have mercy.
Lord give wisdom,
to those who must make decisions.
Lord give your strength,
to those who seek to protect.
Lord give guidance,
to those who must respond.
Lord have mercy,
Christ have mercy,
Lord have mercy."
Six locations in Paris were targeted in these series of attacks.
Jonathan Clarke from Premier Lifeline spoke with Anna Cookson on Big Breakfast.
Listen to their interview here: