Tribal fighters in North Waziristan had vowed to attack security forces after abandoning a 10-month peace pact with the government at the weekend.
'State of emergency'
Pakistan has been rocked by a wave of violence since government troops stormed the Lal Masjid, or Red Mosque, in the capital last week.
A suicide bomb killed 16 people and injured 86 others at a rally in support of the country's chief justice on Tuesday.
The attacks prompted the government to consider imposing a state of emergency on Wednesday.
But Pervez Musharraf, the Pakistani president, later ruled out declaring an emergency.
"The president was asked whether an emergency would be imposed because of the growing violence and the president clearly said it would not," said a senior government official, citing the president in talks with newspaper editors.
An emergency would mean the postponement of elections due around the end of the year. Musharraf has repeatedly said elections, including a bid by him for a second term, would be on time.
Police cordoned off the area in the heart of the capital, Islamabad, on Wednesday where the bombing took place the day before and combed the scene for evidence along with bomb experts.
The bloodshed has heightened tensions with religious radicals calling for more revenge attacks on the government and troops moving into militant strongholds on the border with Afghanistan.
Tuesday's bombing underlined the antagonism as various parties sought to place blame.
Supporters of Iftikhar Mohammed Chaudhry, the former chief justice, have accused the government of being behind the mayhem.
An opposition party, believed to be considering joining a coalition government with Musharraf after year-end elections, said the attack was aimed at its loyalists.
In a sign of mourning and protest, the Pakistan Bar Council, the country's largest lawyers' association, has announced that its members would not engage in any court proceedings on Wednesday except for the chief justice's case.
Chaudhry, whose fight against Musharraf's effort to remove him has fuelled opposition to the president extending his rule, was a few kilometres away when the attacker struck in the evening outside the Islamabad district court building.