Nine police vans and three ambulances are attending the scene in the Dumballs Road area of Cardiff.
A police cordon is in place.
South Wales Police are advising members of the public to avoid the area.
One resident described seeing “armed police” in the area.
A police helicopter has also been seen by residents flying over head.
Twitter user, Beth, wrote: “Anyone know whats happening on Dumballs Road Cardiff?
“Armed police and buildings being told to lockdown.”
A spokesperson for the force said: “We are currently dealing with an incident in the Dumballs Road area of Butetown Cardiff.
A police cordon is in place and more information will be provided as soon as possible.”
Hansen Street is currently closed, however, Dumballs Road is open as normal.
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