The attacks in the two cities occurred Tuesday.
In Quetta, capital of Balochistan province, scene of much deadly sectarian and separatist violence in recent months, a car bomb went off near Nichari road, killing six people and injuring more than 45 others, Dawn reported, quoting police.
On the same day, an explosion in the port city of Karachi killed at least two and injured an undetermined number of people, the report said.
The banned Lashkar-i-Jhangvi claimed responsibility for the attack in Quetta, the report said.
Balochistan has a large population of Shiite Muslims, a minority in Sunni Muslim majority in Pakistan, who have been the target of numerous such attacks lately.
Dawn said the explosion occurred as the vehicle of Abdul Khaliiq Hazara, head of the Shiite sect's Hazara Democratic Party, was passing through the area but was not hurt. He had earlier addressed an election rally.
The explosion damaged more than 10 houses and shops, police said. Hospital officials said at least 10 among the injured were in a critical condition.
The Karachi explosion also occurred near the election camp of a political party at Peoples Chowrangi after which several shops were closed down.
After the attack, Altaf Hussain, head of the Muttahida Qaumi Movement Party, called for a day of mourning in all of Sindh province of which Karachi is the capital, Geo News reported. He also ordered for a temporary closure of all election offices.
The MQM and other secular parties in Pakistan face threats from the Pakistani Taliban, which opposes the elections set for May 11, saying they are designed to divide Muslims. The Pakistani Taliban, which is separate from the Taliban in Afghanistan, is close to al-Qaida.
The civilian coalition government, led by Zardari's Pakistan People's Party, for the first time in Pakistan's history completed its full five-year term last month. The May 11 elections would be an important political milestone as they would mark the first such democratic power transition in a country that has been intermittently under military rule for decades since independence in 1947.