The ban bars citizens from Iraq, Syria, Iran, Libya, Somalia, Sudan and Yemen.
MPs are holding an emergency debate on the immigration measures.
A petition calling for PM Theresa May to cancel Mr Trump's planned state visit to the UK has gathered more than 1.5 million signatures.
The demonstration in London, organised by author and columnist Owen Jones, took place outside 10 Downing Street.
The crowd chanted slogans including "shame on May". Reporters at the scene said there were several thousand people on the streets.