David Cameron has told MPs that bombing so-called Islamic State in Syria will "keep the British people safe" as MPs debated the case for military action.
Labour leader Jeremy Corbyn said the prime minister's case "doesn't stack up" and could make the situation worse.
But shadow foreign secretary Hilary Benn received sustained applause across the House when he urged Labour MPs to "confront the evil" posed by IS.
A 10-hour Commons debate has ended with a vote on endorsing action due shortly.
A separate amendment seeking to block air strikes was defeated by 390 votes to 211.
During the debate, Mr Cameron faced calls to apologise for saying opponents of military action were "terrorist sympathisers", with Mr Corbyn saying it "demeaned" his office.
The PM declined to apologise, but said there was "honour" in voting for or against military action.
Air strikes could begin soon, if the Commons delivers what Mr Cameron hopes will be a majority after Mr Corbyn abandoned attempts to impose his opposition to military action on Labour and allowed his MPs a free vote.
Mr Cameron called on MPs to "answer the call from our allies" and take action against the "woman-raping, Muslim-murdering, medieval monsters" of IS, who he warned were "plotting to kill us and to radicalise our children right now".
He said MPs faced a simple question: "Do we work with our allies to degrade and destroy this threat and do we go after these terrorists in their heartlands from where they are plotting to kill British people, or do we sit back and wait for them to attack us?"
The prime minister also defended his controversial claim that there were 70,000 moderate opposition fighters in Syria, saying it was the estimate of the Joint Intelligence Committee - the UK's senior intelligence body.
He said the majority were members of the Free Syrian Army and that there were a further 20,000 Kurdish fighters with whom Britain could also work.
He told MPs the forces were "not ideal, not as many as we would like, but they are people we can work with".
Mr Cameron also said that in future the UK government would be referring to IS as Daesh as much as possible, because "this evil death cult is neither a true representation of Islam nor is it a state".
Daesh has negative connotations in the Middle East and is seen by some as a way of challenging the legitimacy of the group.
Labour leader Jeremy Corbyn told MPs: "It is impossible to avoid the conclusion that the prime minister understands public opposition to his ill thought-out rush to war is growing - and wants to hold the vote before it slips from his hands.
"Whether it's the lack of a strategy worth the name, the absence of credible ground troops, the missing diplomatic plan for a Syrian settlement, the failure to address the impact on the terrorist threat or the refugee crisis and civilian casualties.
"It's become increasingly clear that the prime minister's proposals for military action simply do not stack up."
He disputed Mr Cameron's claim about ground troops, saying it was "quite clear there are no such forces" and only extremists would take advantage of the strikes against IS.
But shadow foreign secretary Hilary Benn said the UK had a "moral and practical duty" to confront IS, telling MPs there were "rarely perfect circumstances in which to deploy military forces" but the "threat is now" and the UK must rise to the challenge.
Islamic State, he told MPs, were "fascists and fascists must be defeated". At the end of his 10-minute speech, Mr Benn was roundly applauded on the Conservative benches, although the reception among Labour MPs was more mixed.
Three former Labour ministers - Alan Johnson, Dame Margaret Beckett and Yvette Cooper - also made speeches in favour of extending military action.
Mr Johnson, a former home secretary, said he believed IS had to be "confronted and destroyed if we are to properly defend our country and our way of life".
And he took a swipe at Jeremy Corbyn's supporters, saying: "I find this decision as difficult as anyone to make, I wish I had frankly the self-righteous certitude of the finger-jabbing representatives of our new and kinder type of politics, who will no doubt soon be contacting those of us who support this motion tonight."
Jeremy Corbyn's aides had said as many as 90 Labour MPs could vote to back the government motion, although there is speculation it may actually be between 40 and 50.
Shadow home secretary Andy Burnham told BBC News he would be voting against air strikes as he believed they would make an attack in the UK more likely but he added: "I wouldn't rule out taking military action at some point."
Former Labour leader Ed Miliband will also vote against air strikes, telling the LabourList website they would not "defeat ISIL or make us safer here at home".
So far 11 Conservatives have said they oppose the prime minister's position.
Former shadow home secretary David Davis said allied air strikes against IS targets had achieved the "opposite" of their intended effect and the number of recruits to Daesh had doubled to 30,000 since they began - "one extra recruit for every target".
He said it was "debatable" whether allowing the RAF to strike targets in Syria "will make any difference at all" to the military effort.
Conservative MP John Baron warned MPs that "without a comprehensive strategy, air strikes will simply reinforce the West's long-term failure in the region".
Angus Robertson, the SNP's leader at Westminster, said: "I appeal to colleagues on all sides to make sure that we do not ignore the lessons of Afghanistan, ignore the lessons of Iraq, ignore the lessons of Libya.
"Let's not repeat the mistakes of the past, let's not give the green light to military action without a comprehensive and credible plan to win the peace."
But Lib Dem leader Tim Farron warned against learning the wrong lessons from the "illegal, counterproductive war in Iraq", saying: "On balance it is right to take military action to degrade and defeat this evil death cult."
By BBC Security Correspondent Frank Gardner
Independent experts say the figure of 70,000 is broadly accurate but there is a high degree of scepticism as to how many could actually be persuaded to fight IS instead of the Syrian regime, let alone be moulded into a cohesive, effective force.
There are also questions over just how "moderate" and pluralistic many of the Islamist factions are within this number. It is far from certain that, if empowered in a post-Assad, post-IS Syria, battle-hardened Sunni rebels would be happy to share power with Christians and Alawites.
At least 110 MPs from six different parties - including the SNP - signed up to an amendment seeking to block air strikes but this was defeated by 390 votes to 211.
Anti-war protesters staged a demonstration outside Parliament as MPs debated the issue, with one woman crawling under a lorry and refusing to move. She is reported to have been arrested.
The prime minister caused controversy on the eve of the vote by labelling Mr Corbyn and other opponents of action as "terrorist sympathisers".
Mr Benn criticised the prime minister for "failing to saying sorry" for the remarks, telling MPs that while he took a different view from Mr Corbyn on bombing Syria, the Labour leader was a "good and principled man".
Facing calls to apologise, Mr Cameron said: "Everyone in this House should make up their mind on the arguments in this House and there's honour in voting for, there's honour in voting against.
"That is the way this House should operate and that's why I wanted to be absolutely clear at the start of my statement that this is about how we fight terrorism not whether we fight terrorism."
The government says military action is "only one component of a broader strategy" to tackle IS and the UK government would not deploy troops on the ground.
The UK is already providing intelligence, surveillance and other logistical support to countries fighting IS in Syria. The RAF has also carried out thousands of raids on IS targets in Iraq since Parliament approved similar action there last year.