The biggest rally took place in London where thousands of demonstrators urged Tony Blair, the British prime minister, to stop what they described as his refusal to condemn Israel's actions and join international calls for an immediate ceasefire.
"Peace for Lebanon!" they chanted as the march weaved its way through central London, past the US embassy and on to Hyde Park, watched all the way by the police.
"Stop the killing, stop the bombs. Israel out of Lebanon," shouted the peaceful protestors, many draped in Lebanese or Palestinian flags.
Others shouted "Hezbollah is here to stay. Zionism go away".
Betty Hunter, the general-secretary of the Palestine solidarity campaign, one of the groups that organised the event, said it was vital to reject Israel's two-pronged campaign against Hezbollah targets in Lebanon and Hamas in Gaza.
'Ashamed' of Blair
She said: "The main purpose of this demonstration is to say to Tony Blair and our government that we are ashamed of the position they are taking which is basically to collude with the war crimes of Israel."
Israel's 11-day air offensive in Lebanon has left more than 372 Lebanese and 34 Israelis dead, while more than 100 Palestinians and one Israeli soldier have died in Gaza.
People hold Lebanese flags at a
march in London
The operation in Lebanon was sparked when the Shia group Hezbollah kidnapped two Israeli soldiers on July 12.
Disgust at the toll spurred people to join the rally in London.
Police put the number of participants at 6,000 to 7,000, while organisers said the turnout was between 20,000 and 25,000.
A similar show of solidarity with Israel is planned near London on Sunday evening and will be addressed by Britain's chief rabbi.
"This war is a catastrophe. We can prevent this catastrophe through negotiations that would save the lives of Arabs and Israelis"
Mohammed Barrakeh, an Israeli Arab parliamentarian
Henry Grunwald, who heads the board of deputies of British Jews, said: "Israel has the right to defend itself against unprovoked attacks on sovereign soil."
Much of the anger at Saturday's London protest was directed at the British government for its refusal to openly condemn Israel's actions and call for an immediate ceasefire.
Yasmin Ataullah, the spokeswoman for the British Muslim initiative, said: "We're disgusted by the way the US and Britain have isolated themselves from the rest of the international community."
Speaking in Beirut, Kim Howells, a foreign office minister, made the strongest criticism yet of Israel by a British government minister.
He said: "These have not been surgical strikes. It's very, very difficult to understand the kind of military tactics that have been used.
"You know, if they're chasing Hezbollah, then go for Hezbollah. You don't go for the entire Lebanese nation."
'No war' in Sydney
In Sydney, a 10,000-strong crowd waved Australian and Lebanese flags and carried coffins and placards saying "No War" as they made their way through the city centre, escorted by about 400 police.
Students, socialists and peace
activists march in Sydney
"They are murdering children and burying them under rubble," a Lebanese-Australian woman, who gave her name only as Diana, told Australian associated press.
Jews and Arabs in Tel Aviv
In Tel Aviv, 1,000 Israeli Jews and Arabs turned out to denounce their country's actions, gathering in Rabin Square and brandishing placards reading "war is disaster" and "Jews and Arabs refuse to be enemies".
Mohammed Barrakeh, an Israeli-Arab MP, said: "This war is a catastrophe. We can prevent this catastrophe through negotiations that would save the lives of Arabs and Israelis.
"The war won't end soon but we won't stop protesting either."
A recent opinion poll showed that 95% of Israelis support the military offensive and the popularity of Ehud Olmert, Israel's prime minister, has shot up since it began.
Geneva's silent march
In Stockholm, where 2,000 marched to the Israeli embassy, several hundred protestors clashed with police, throwing stones and objects at police officers. Two people were arrested.
Other demonstrations took place in Geneva, Paris, Strasbourg, Warsaw, Chicago, Amsterdam and in a number of cities around Britain.
In Geneva, 500 people marched in silence behind a coffin meant to symbolise the death of the conscience of the United Nations.
Anouar Gharbi, the president of the rights for all association that organised the protest, said: "We have chosen a silent march to show that there is no word to qualify the unqualifiable."
And several hundred demonstrators gathered in downtown Chicago carrying banners that read: "The Right to Fight Or The Might to Smite", or "Not with our money, not in our name."
"We have chosen a silent march to show that there is no word to qualify the unqualifiable"
Anouar Gharbi, president of rights for all, Geneva
'Outraged' in Chicago
Dale Lehman, a 60-year-old Jewish resident of Chicago, said: "I'm outraged as an American, I'm outraged as a human being at what is happening to the people of Lebanon."
A small counter-protest demonstrated in support of Israel over the road from the main rally.
The main Chicago rally was organised by the American council on American-Islamic relations.