[cnn-photo-caption image=http://i2.cdn.turner.com/cnn/2009/WORLD/asiapcf/05/29/pakistan.peshawar.blast/art.lahore.amath.afp.gi.jpg caption="Pakistanis look Thursday at the rubble of a police building in Lahore hit by a suicide bomb on Wednesday."]
Pakistani authorities increased security throughout the capital Friday after this week's deadly bombings in Lahore and Peshawar, and a threat by the Taliban to carry out further attacks.
All vehicles must go through checkpoints before entering Islamabad, the city's deputy police inspector general Bin Yamin told CNN. Three people have been arrested in Islamabad in connection with the current threats, he said.
The Pakistani Taliban claimed responsibility for Wednesday's suicide attack in Lahore that killed 27 people. It also threatened to continue attacking cities in Pakistan until the military ends its operations against Taliban militants in the country's northwest.
There has been no claim of responsibility for the Thursday attacks in North West Frontier Province.
Officials said they believed bombers who killed 12 victims Thursday may have caused some of the blasts to lure people nearer before detonating further explosions.
The attacks targeted Dera Ismail Khan and the provincial capital, Peshawar, where government forces have waged a massive operation against Taliban militants.
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