Emergency services were called to reports of a loud bang in Clark Mount, Richmond Hill, shortly after 6.50pm.
No one was injured in the incident but a number of neighbouring properties were evacuated and roads closed.
Today police said it appeared that a bag containing fireworks had been hurled through an open window at the ground floor flat.
Army bomb disposal experts were called in to inspect the bag as a precaution before the evacuated residents were allowed back into their homes at about 10pm.
Leeds District CID’s Acting Det Insp James Entwistle said: “The nature of this incident is clearly a cause for concern and we are carrying out extensive enquiries to establish the full circumstances and trace those responsible.
“There were a number of people in the property at the time but thankfully no-one was injured although the consequences could have been much worse.
“We have already spoken to a number of witnesses but we would like to hear from anyone who saw anyone acting suspiciously in the time leading up to the incident or who has any information that could assist the investigation.”
Anyone with information is asked to contact Leeds District CID at Elland Road via West Yorkshire Police’s 101 number or Crimestoppers anonymously on 0800 555111.