Officers were scrambled to the “ongoing and dynamic” incident at 1.35pm today.
The A1 at the Wallyford slip road remains closed for scene preservation as forensic officers comb the area.
Details on the nature of the incident are yet to be released as police contine enquiries.
Local resident and commuters reported lengthy tailbacks in the area.
Lothian Buses have diverted services, 104, 106 and 113 via Church Street, Bankton Terrace and Mid Road in both directions since 3.30pm.
A Police Scotland spokesman said: “Police in East Lothian were called to a report of a disturbance at 1.35pm.
Enquiries are ongoing to establish the full nature of the incident and officers, including those from operation support division, are in attendance.”