in the southwestern city of Quetta, and a car bombing killed 17 in the northwest.
The combined toll marked the deadliest day for more than four months in the nuclear-armed state, which is battling homegrown extremism and is on the frontline of the US-led war against Al-Qaeda.
In Quetta, officials said a suicide bomber tried to access a Shiite mosque in the congested Hazara town, a mainly Shiite community on the edge of the city, but was intercepted at a checkpoint.
"Twenty-eight people have been killed and more than 51 others are injured," said senior police official Mir Zubair.
"Nine women, a minor girl and four boys were also killed in the attack," Zubair told reporters.
Baluchistan provincial home secretary Akbar Hussain Durrani said the bomber blew himself up at a checkpoint set up by a local neighbourhood volunteers around 50 yards away from the mosque.
"The evening prayers had just ended in the mosque and most of the victims are Shiite Muslims," he told AFP.
Quetta is one of the most volatile cities in Pakistan, gripped by Taliban violence, a separatist Baluch insurgency and a growing number of attacks on the Shiite minority.
Hazara Town in particular has been a flashpoint for attacks on Shiites and the minority Hazara ethnic group.
Earlier, while Cameron was holding talks with new Prime Minister Nawaz Sharif in Islamabad, a car bomb targeted a security force convoy on the outskirts of Peshawar, not far from the semi-autonomous tribal belt where Taliban and Al-Qaeda-linked groups have bases.
Jamil Shah, spokesman for the government-run Lady Reading Hospital in the city, said 17 people were killed and 46 injured.
At least four children and one woman were among the dead, and two children and a woman were among those hurt, he added.
Police said most of the victims were civilians because the bomb targeting the Frontier Corps (FC) convoy exploded in a bustling market area.
Shops and cars were damaged in the attack, an AFP reporter said. Pieces of human flesh, broken glass, lost shoes and vegetables from nearby carts were flung across the scene, and the seats of damaged cars were stained with blood.
A military official told AFP that two Frontier Corps soldiers were injured, but that "we don't know about any other losses".
There was no immediate claim of responsibility but the Pakistani Taliban frequently targets security forces as part of a seven-year domestic insurgency that has killed thousands of Pakistanis.
Terror plots against the West have been hatched in Pakistan's semi-autonomous tribal belt in the northwest, where Pakistani troops have for years been fighting homegrown militants.
Speaking in Islamabad, Cameron said the battle against terrorism needed "a tough and uncompromising security response" as well as investment in education and measures to ease poverty.
Sharif said Pakistan was "resolved to tackle the menace of extremism and terrorism with renewed vigour and close cooperation with our friends".
In the northwestern tribal district of South Waziristan, a roadside bomb killed four people in the main town of Wana, officials said.
In the North Waziristan tribal region, another roadside bomb targeting a security forces convoy killed four security officials and wounded 12 in the main town of Mir Ali, officials told AFP.
Pakistan's government, which took office in June after historic elections, faces a massive array of problems including a moribund economy and Islamist militancy.
On June 22, gunmen shot dead 10 foreign tourists and a Pakistani guide in an unprecedented attack at a remote base camp in the Himalayas that was claimed by Pakistan's umbrella Taliban faction.
As a result Pakistan suspended expeditions to its second highest peak, Nanga Parbat, and the tiny tourist industry built around natural beauty and mountaineering opportunities is set to collapse.
Pakistan's Taliban movement claimed responsibility on behalf of Junood ul-Hifsa, a new faction it said had been set up to kill foreigners to avenge US drone strikes on Taliban and Al-Qaeda operatives in the tribal belt.
Sharif has previously advocated peace talks with the Taliban and criticised the drone strikes, echoing long-held complaints that the US campaign violates national sovereignty.
Sunday was the deadliest day in Pakistan since February 16, when a giant tractor bomb killed 90 Shiites from the ethnic Hazara minority in Quetta. On January 10, 92 others were slaughtered by suicide bombers at a snooker hall elsewhere in the city.