Around 1,000 EDL protesters have made their way along Whitehall before taking a position on the pavement across the road from Downing Street.
A much smaller group of anti-fascist demonstrators gathered opposite them holding an "EDL Racists" banner.
Both sides traded insults but were kept apart by officers and barriers.The EDL supporters chanted: "We hate Cameron."
More top news
The Gambling Commission said: "Integrity in sport is not a joke and we have opened an investigation to establish exactly what happened."
The landmark development will see all London staff working under one roof but will mean the closure of three historic studios.
Five activists were arrested after staging a three-hour demonstration which forced the closure of a tunnel at Heathrow Airport.