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At least 20 people have been killed in an attack targeting the Kabul office of vice-presidential election candidate Amrullah Saleh.

Fifty other people were injured in the attack on the Green Trend office in the capital on Sunday.

The violence came on the first day of campaigning for Afghanistan's upcoming presidential elections.

Mr Saleh, who was slightly injured, is the running mate of President Ashraf Ghani.

The attack saw a suicide bomber detonate a car packed with explosives near the office building, said a spokesman for the interior ministry.

Several militants then entered the office and remained for about six hours before being killed by security forces.

About 150 civilians were rescued from inside the building where Green Trend - a social, grassroots and anti-corruption organisation led by Mr Saleh - is located.

President Ashraf Ghani had earlier on Sunday said that "peace was coming" to the nation.

No militant group has claimed responsibility for the attack.

Afghanistan's election is expected to take place in September, but has already been postponed twice.