Amid tight security, hundreds of Pakistanis have paid their respects at a mass funeral for 45 minority Shiites killed in an attack the day before on a bus in the southern city of Karachi.

Pakistan is observing a day of mourning Thursday over the killings. State-run television is broadcasting live footage showing mourners attending the last rituals for the victims. Gunmen had stormed the bus with the Shiites on Wednesday, ordered them to bow their heads and then shot them dead.

It's unclear who was behind the assault. Both a Pakistani Taliban splinter group and an Islamic State affiliate claimed responsibility for it. Pakistani Sunni militant groups view Shiites as apostates and have targeted them in the past.

Prime Minister Nawaz Sharif had rushed to Karachi and ordered an investigation.