Most of the casualties were passersby and traders working near the Weheliye hotel on the busy Makka Almukarramah road.
Police said emergency services were responding to the incident, but there was no risk of additional explosions.
Police say the explosion which occurred near a high-traffic thoroughfare was caused by a suspected suicide bomber. Officials say that is expected to rise as dozens were injured.
The explosion left many seriously wounded who are getting treatment at a local hospital.
Dozens of labourers were on site at the factory when a boiler exploded at the six-storey building, causing its walls and a roof to collapse.
Troops patrolling Brussels Central Station "neutralised" a person after a small explosion. No one else was injured.
Police made a preliminary judgment that the blast is a "criminal offence" and said that the suspected bomber also died in the explosion.
The nature of the blast is not clear, but an official at the police station in Fengxian county said that the explosion was under investigation.
A homemade explosive device that went off during a patrol in a Shia town also wounded two soldiers, the Saudi interior ministry said in a statement, describing it as a "terrorist incident."
The explosion that also injured 10 people occurred after a fire ignited gunpowder stocks at the factory in Madhya Pradesh state.
Fans watching the Champions League final on a giant screen in San Carlo Square ran in panic when they mistook firecrackers for an explosion.
Explosions ripped through the funeral of a demonstrator who was killed in clashes with security personnel during anti-government protests.
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