Lawmaker Khan Mohammad Wardak survived the blast but is among 20 injured including women and children, Afghan Interior Minister Massoud Andarabi said.
The Taliban denied their fighters were involved in the assault, and the identity of the attackers was unknown
A spokesman for Helmand's governor said several rockets fired by Taliban insurgents landed close to the cattle market, killing 23 civilians.
"It is intolerable, assassinating human rights defenders is a war crime in the context of armed conflict," the AIHRC said, adding that staff had previously been the target of attacks.
The interior ministry in Kabul and provincial officials in Ghazni confirmed the attack.
Officials say a deal between the Ghani and Abdullah is crucial to launching peace talks, as Abdullah's camp represents much of the country's north-west.
A Ministry of Interior statement confirmed an attack had taken place at a hospital.
In the run-up to the pact, there was a one-week reduction in violence, but attacks and bombings have continued since, with the Taliban having rejected calls for a ceasefire.