One gang is thought to be behind violent racially-motivated attacks on Asian men in an underpass in Redbridge.
The first attack was on 29 September when a 35-year-old man was surrounded and punched in the face.
Five days later, a 33-year-old man was confronted by up to 15 people who punched him, fracturing an eye socket.
In the most recent incident two men in their 20s were surrounded. One was assaulted and verbally abused, while the other managed to escape uninjured.
Officers, who believe there may have been other similar unreported incidents, have stepped up patrols in the area of the underpass near Somerville Road beneath the A12.
The youths in the gang are mainly white but some blacks males were also involved.