Police were called to a pub in London's Soho after a "noxious substance" was released inside.
A Metropolitan Police spokesman said 10 people were left coughing and spluttering after the incident but none were seriously injured.
Officers were at Comptons pub in Old Compton Street on Wednesday night to try to identify the substance released.
The spokesman said no-one had been arrested and officers were keeping an open mind as to the motive.
The emergency services were alerted at about 1930 BST.
Police said no-one had been able to confirm what had happened.