A massive suicide bombing in central Lahore targeting Pakistan security services has left more than 40 people dead and hundreds injured.

The Pakistan government immediately declared a state of emergency after the bomb, detonated at 10.35am local time, demolished the three-story Lahore Inter-Services Intelligence (ISI) agency building, sending a large mushroom-shaped cloud over the city.

Other security agency buildings destroyed in the blast include the Lahore Capital City Police Office (CCPO), the largest police building in the country, and the office off the country’s Rescue 15 service.

Pakistan security forces detained three people at the scene who were attempting to dispose of weapons and blend in with people rushing to the scene to assist.

As many as four terrorists remain inside the damaged CCPO building, exchanging gunfire with Pakistan security forces outside, as a massive security cordon was put into place in an attempt to capture or neutralize them.

The blast occurred just 200 meters from the Punjab Assembly and at the same time as Hafiz Mohammed Saeed, head of the banned terrorist group Lashkar-e-Taiba (LeT) was due to be produced in the Lahore High Court charged with terrorism.

A major Pakistani military offensive is also currently underway against Taliban militants in the Swat valley in the north-west of the country leading many to speculate the Taliban may have been behind today’s suicide bombing.

Pakistan Interior Minister, Rehman Malik, said the country was “in a state of insurgency” and that “there will be no negotiations” with terrorists holed up in the CCPO building.

Lahore hospitals were swamped with a constant stream of injured from the blast site and all off-duty medical staff were recalled. Hospitals issued urgent appeals for blood donations from the public and all patients waiting for non-essential surgery were sent home.

The government also closed Lahore schools and a number of children in schools close to the scene of the bombing are reported to have been injured.

Officials in Lahore expect the death-toll to rise as rescuers and volunteers, many digging with their bare hands, attempt to remove concrete, steel and debris from the bombed buildings.

This is a breaking news story. Check back for updates