The city has seen more than 100 new cases in the fresh outbreak, the World Health Organization (WHO) says.
For more than seven weeks Beijing had only registered cases from people travelling in from abroad.
New clusters of coronavirus are "always a concern", said Mike Ryan, the WHO's emergencies programme head.
"But what we do like to see is an immediate response to that and comprehensive set of measures," he added.
What do we know about the new cases?
The fresh outbreak has been linked to city's largest wholesale market, Xinfadi.