A bomb hidden in an ambulance killed at least 40 people and wounded about 140 at an Afghan police checkpoint in an area of Kabul near foreign embassies and government buildings, officials said.
"The suicide bomber used an ambulance to pass through the checkpoints. He passed through the first checkpoint saying he was taking a patient to Jamuriate hospital and at the second checkpoint he was recognized and blew his explosive-laden car," AFP quoted interior ministry deputy spokesman Nasrat Rahimi as saying.
The Taliban claimed responsibility for the blast.
“It is a massacre,” said Dejan Panic, coordinator in Afghanistan for the Italian aid group Emergency, which runs a nearby trauma hospital. In a message on Twitter, the group said more than 50 wounded had been brought in to that hospital alone.
Health ministry spokesman Wahid Majroh said at least 40 people were killed and 140 wounded in the blast, which blew up in a crowded street in a busy part of the city at lunchtime.
Saturday is a working day in Afghanistan.
Buildings hundreds of meters away were shaken by the force of the explosion, which left torn bodies strewn on the street nearby amid rubble and debris.
In chaotic scenes at the Jamuriate hospital, which is the nearest medical facility to the blast, overwhelmed doctors and nurses rushed to treat dozens of wounded lying in the corridors.
Outside civilians walked through debris-covered streets carrying wounded people on their backs as paramedics loaded several bodies at a time into ambulances to take them to medical facilities around the city.
The explosion comes exactly a week after Taliban militants stormed a luxury hotel in Kabul, killing at least 22 people, the majority foreigners.
A security alert issued to foreigners on Saturday morning warned that the Islamic State group, which has terrorized the city in recent months, was planning "to conduct aggressive attacks" on supermarkets, shops and hotels frequented by foreigners.
The latest attack will add pressure on President Ashraf Ghani and his US allies, who have expressed growing confidence that a new more aggressive military strategy has succeeded in driving Taliban insurgents back from major provincial centers.
The United States has stepped up its assistance to Afghan security forces and increased its air strikes against the Taliban and other militant groups, aiming to break a stalemate and force the insurgents to the negotiating table.
However, the Taliban have dismissed suggestions that they have been weakened by the new strategy and the latest attacks have demonstrated that their capacity to mount deadly, high- profile attacks remains undiminished.