Thousands of demonstrators took to the streets of the Swedish city on Friday to protest against capitalism and the European Union.
Misguided
But Mr Blair criticised the protests, saying the demonstrators were "completely misguided".
Mr Blair backed the Swedish authorities trying to keep the protesters under control.
"I want to express my sympathy for the Swedish authorities and the Swedish police for what they have had to put up with and my sympathy for the police who have been injured.
"These protests are a complete outrage.
"World trade is good for people's jobs and people's living standards.
"The protests and people who indulge in the protests are completely misguided."
Foreign Secretary Jack Straw also condemned the rioters.
"There is no justification for what's been going on outside," he said.
"Violent demonstrations have no place in a democracy."
Anti-globalisation and anti-EU protesters threw fireworks and stones at police, who fought back with clubs.
Some mounted police were thrown off their horses.
Police sealed off a shopping mall in the centre of town after a suspected bomb was found.
Demonstrators tried repeatedly to get close to the conference hall where the 15 EU leaders were meeting, and at one stage were reported to be less than a mile away.
Several hundred people were arrested on Thursday after similar clashes during President George W Bush's meeting with EU leaders.