Britain's terror threat level is "severe", but Prime Minister David Cameron said it would reviewed.
Police said there would be strengthened policing at UK ports, and more officers at public events in the coming days.
It comes as Briton Nick Alexander, from Colchester, was named as among the 129 people killed in Friday's attacks.
The 36-year-old died in the attack at the Bataclan concert hall, where he is thought to have been selling merchandise.
Read all the latest updates on the Paris attacks
His family described him as "generous, funny and fiercely loyal".
Friends of Mr Alexander also paid tribute to him on social media. Joe Trohman, lead guitarist of the rock band Fall Out Boy described him as a "great guy".
The Bishop of Chelmsford, the Right Reverend Stephen Cottrell, said he was praying for Mr Alexander's family and had sent a message of support to their local church.
A government source said there were fears a "handful" of other British people had been killed. An unspecified number are being treated in hospitals.
The Bataclan concert hall suffered the deadliest attack during the wave of violence in Paris, which involved gunmen and suicide bombers. More than 80 people are believed to have died at the concert hall.
The US band Eagles of Death Metal were playing a gig when attackers burst into the venue and opened fire, but the band themselves survived unscathed.
Michael O'Connor, from South Shields, was at the concert hall when the gunmen attacked.
The 30-year-old lay on top of his girlfriend to protect her as other audience members were shot and killed. Both escaped unharmed after playing dead.
"What they want to do is to terrify us," he told BBC Radio 5 live's Stephen Nolan show.
"We can't let them win and these people just, they're just monsters, they're just, they don't represent anything, they're not Muslims, they're not, they're just animals.
"How can you walk in there, I mean, I'm 30 years old, I was probably the oldest, one of the oldest people in the gig.  It was full of teenagers and, you know, people in their early 20s going to see a band."
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In other developments:
So-called Islamic State (IS) has said it carried out the attacks, and Mr Cameron said it showed the threat from the group was "evolving", with a "new degree of planning and co-ordination and a greater ambition for mass-casualty attacks".
He added that the group's "aim is clear: it is to divide us and to destroy our way of life.
"So, more than ever, we should come together and stand united and carry on with the way of life that we love."
French President Francois Hollande said the attacks were an "act of war" by IS.
Talks about the security threat are expected later after leaders, including David Cameron, from the G20 countries meet in Turkey.

The Iraq Historic Allegations Team (Ihat) has decided not to proceed in 57 cases, the MoD said.
A further case was stopped by the military's prosecuting authority.
The news follows a call by PM David Cameron to "stamp out" what he called "spurious" legal claims against British troops returning from  action overseas.
Ministers had been asked to draw up plans to curb claims, including by restricting "no win, no fee" arrangements, Mr Cameron said.
Lawyers say no-one is above the law, and many abuse cases have been proven.
Ihat was set up to review and investigate allegations of abuse made by Iraqi civilians against UK armed forces personnel in Iraq during the period of 2003 to July 2009.
It currently lists more than 1,300 allegations under investigation, ranging from murder to low-level violence.
Conservative MP Richard Benyon, a member of the Commons Defence select committee, said innocent veterans were being unfairly targeted.
He told The Sun: "It's an intolerable burden for people who have served their country well to face this knowing they're innocent."
Home Secretary Theresa May will chair a Cobra meeting later to discuss the UK government's response to the attacks in Paris.
[NEXT_CONCEPT]
Investigations into nearly 60 allegations of unlawful killing against UK soldiers in Iraq have been dropped, the Ministry of Defence says.