The 16 Egyptians and four foreigners are due to appear in court tomorrow, accused of backing Egypt's blacklisted Muslim Brotherhood.
Demonstrators in London held placards and taped over their mouths during the protest staged by the National Union of Journalists.
Among those detained is former BBC journalist Peter Greste.
Sue Turton, a former presenter for Britain's Channel 4 television who now works for Al-Jazeera, described Greste as a "measured and balanced journalist" and called on the judges to do the right thing and release them after 53 days of detention.
More than 38,000 people have signed a petition started by Greste's parents, and the global campaign movement Avaaz.
The petition will be handed in to the Egyptian government before the trial begins. If found guilty, Greste could face seven years in prison.
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