ISLAMABAD, Pakistan  A suicide bomber blew himself up at the entrance to a crowded Shiite mosque just south of the capital on Sunday, killing at least 26 people. It was the third suicide attack in Pakistan in 24 hours, in a sign that the Pakistani Taliban are overwhelming the nation’s security forces.

The assault south of the capital, Islamabad, appeared to be carefully planned. It took place in Chakwal, a town that has historically had strong ties to the Pakistani Army, and in a Shiite mosque. The Pakistani Taliban have increasingly attacked Shiite mosques.

The latest bombing occurred about 12 hours after a suicide bomber struck in an upper-class neighborhood of Islamabad on Saturday night, killing eight paramilitary security officers assigned to guard foreign diplomats and wealthy residents. On Saturday morning, a suicide bomber drove his vehicle into a group of civilians on the side of the road in Miram Shah, in North Waziristan, killing at least eight people, including schoolchildren.

In a telephone interview on Sunday, Hakimullah Mehsud, a powerful deputy to Baitullah Mehsud, the leader of the Pakistani Taliban, said the Taliban were responsible for the suicide attacks in Islamabad and Chakwal.