a popular gathering place for politicians, foreigners and the Pakistan elite. As well as killing at least 60 people, it wounded about 200 others.
US President George W Bush said in a statement that "the attack is a reminder of the ongoing threat faced by Pakistan, the United States, and all those who stand against violent extremism."
He vowed to "fully support the democratically elected government of Pakistan and the Pakistani people as they face enormous challenges economically as well as from terrorism."
Afghan President Hamid Karzai, who has long been at odds with Pakistan on the policing of the two countries' shared border, called for an "honest struggle" to beat terrorism.
"As long as this evil phenomenon is active in the region, terror, horror and explosions will continue," he said in a statement.
Indian Prime Minister Manmoahn Singh wrote a letter to his Pakistani counterpart Yousuf Raza Gilani, to express his "shock and sorrow" over the attack.
"We strongly condemn this act of terrorism," wrote Singh.
He added: "Such attacks are a grim reminder of the challenges we face from destructive forces which have pitted themselves against the values of democracy and pluralism and the voices of moderation," he wrote.
"We must not allow such forces to succeed."
India and Pakistan have themselves fought three wars in the past, and Delhi's anti-terrorism chief Karnal Singh on Sunday accused Pakistan's spy agency of being behind a recent wave of bombings in India.
A statement issued by UN Secretary General Ban-Ki Moon described the bombing as a "heinous terrorist attack".
He expressed his "heartfelt condolences to the families of the victims of this appalling attack, as well as to the government and people of Pakistan. No cause can justify the indiscriminate targeting of civilians."
Czech Foreign Minister Karel Schwarzenberg denounced the bombing, which claimed the life of Prague's recently appointed ambassador to Pakistan, Ivo Zdarek.
"His death shows that the terrorists are trying to hit our most valuable spots. We will not waver from out path: we will always stand up to evil and combat evil," Schwarzenberg said.
Ekmeleddin Ihsanoglu, the secretary-general of the Saudi-based Organisation of the Islamic Conference (OIC) described the attacks as "despicable terrorist actions completely contradict the praiseworthy precepts of Islam."
He called the perpetrators "the enemies of peace whom the international community must fight with all possible means."
There has been no claim of responsibility for the bombings but investigators in Islamabad say it is likely they were carried out by Al-Qaeda.