The Hollywood couple had spent the day at royal residence Buckingham Palace, where Zeta-Jones was honoured as a Commander of the Order of the British Empire (CBE) by Charles, Prince of Wales.
The stars were chauffeured back to their hotel in the British capital, but they encountered trouble as they exited their car and had to fight their way through a horde of snappers waiting at the entrance to their residence.
In video footage of the brawl, obtained by U.S. news show Entertainment Tonight, Zeta-Jones can be heard shouting, "How dare you punch me!" at one overzealous paparazzo as they enter the hotel lobby.
She then demands, "I want a police officer right now. He punched me! The guy coming in here, he punched me!"
Douglas leaps to his wife's defence and stares menacingly at the alleged attacker, branding him an "a**hole" before appearing to grab the photographer and push him as hotel security rush to separate the two men.
It is not known whether an official police complaint was filed over the incident, which spoiled a "fabulous" day for the Douglases, according to the Wall Street star's publicist Allen Burry.
Referring to the video, he tells Entertainment Tonight, "You saw it for yourself. Otherwise, the trip and the ceremony were fabulous and such a joy for everyone."