An hour after a suicide bomber blew himself up inside the sports hall in a heavily Shiite neighbourhood, a vehicle packed with explosives detonated as journalists and security forces gathered at the scene, police spokesman Hashmat Stanikzai said.
A spokesman for the interior ministry said at least 20 people were killed.
While the Taliban reportedly denied responsibility, the attack bore the hallmarks of the terrorist group ISIL, also known as Da'esh, which has carried out a wave of bombings against minority Shiites in recent years.
A regular training session in Kabul turned into a massacre when a suicide bomber shot dead a young, unarmed guard at the entrance before blowing himself up near the scores of wrestlers, some of whom were as young as 10.
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Mohammad Jawad Ghawari, a member of Afghanistan's council of Shiite clerics, said there have been 15 attacks against Shiites in Kabul and nine more elsewhere in the country in just the last two years. There are far more insider attacks on Afghan security forces than on Western forces. IS said it was behind that attack, which drew worldwide condemnation and came amid a wave of deadly violence across the country.
Wednesday's attacks resembled one conducted months ago outside an intelligence agency in Kabul where journalists covering the first attack fell victim to the second blast.
In mid-August, at least 34 people were killed in a suicide attack in front of an education centre in Dasht-e-Barchi, west of Kabul.
The Balkh journalists said such incidents should be taken seriously by government and that perpetrators should be arrested and prosecuted.