Tony Blair has said the situation in Gaza offers "no justification" for the radicalisation of British Muslims.
The former prime minister, who is now a Middle East peace envoy, was speaking to the BBC on the eve of a large pro-Palestinian rally in London.
Tens of thousands are expected at the demonstration organised by the Stop the War Coalition.
Mr Blair said protest is legitimate, but the situation does not give anyone any right to commit acts of terror.
He said: "Whatever people feel about what is happening in Gaza - and however strongly people feel about it - there is absolutely no justification for somebody in the UK, who's a UK citizen, that has all the freedom and the opportunities that UK citizens have, saying 'this gives us some right to commit acts of terrorism, or to start agitating in an illegitimate way against the government here'."
He said he knew about "protests here about what's happening in Gaza, that's a perfectly democratic, legitimate way of making clear what your view is - but it offers no justification at all for people doing things that are wrong or criminal here."
Almost 800 Palestinians are thought to have died in the two-week conflict
His comments came as activists were planning a major pro-Palestine demonstration.
Organisers predict thousands of people will join the march through central London on Saturday in protest at Israel's attacks on the Gaza Strip in defiance of a United Nations resolution calling for an immediate ceasefire.
Celebrities including musician Brian Eno and film star Juliet Stevenson are expected to join in the procession from Hyde Park to the Israeli embassy in Kensington.
The organisers - principally Stop the War Coalition, the British Muslim Initiative and the Palestine Solidarity Campaign - said they hoped there would be no repeat of the clashes with police that affected a demonstration in London last Saturday.
Lindsey German, Stop the War's convenor, said she hoped the march would be a passionate but peaceful protest
She added: "We want the British government to take a much stronger position. There would have been outrage from governments around the world if this had happened anywhere else - the condemnation has been at best half-hearted."
Mr Blair, speaking on the BBC's Newsnight programme, said he had hoped a ceasefire could have been achieved by this time.
He said: "We're going to do what we can, we will see whether it bears fruit or not. I'd hoped very much that we would have have got a ceasefire by now. It's not been possible.
"I think there are still real issues about what can be done about the smuggling of the arms into Gaza and then the opening of the crossings so there can be proper humanitarian help and Gaza can be reopened to the outside world on the other side of it.
"At the moment it seems to not be in a position where we are going to get the ceasefire that we want but we are going to carry on working for it, we've got no option."
On Sunday morning the Board of Deputies of British Jews (BoD) will hold a rally in London's Trafalgar Square calling for peace.
Its officials have reacted angrily after a hoax e-mail claimed that the rally had been cancelled.
The BoD denounced the "hurtful" hoax and said it was particularly problematic because the e-mail had been sent before the Jewish sabbath, during which religious Jews do not use electrical appliances.
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