The chief executive of anti-monarchy group Republic, who was also among the 52 people arrested on Saturday, spent 16 hours in police custody.
Mr Smith said a total of eight members of Republic’s team were arrested as they prepared for “a peaceful and lawful protest”.
He said: “I have been told many times the monarch is there to defend our freedoms. Now our freedoms are under attack in his name.”
Asked what further action he and other members of Republic face, Mr Smith said: “We don’t know. We’ve all been bailed, that’s all we know. So, we will hear about whatever happens later.”
The arrests come after the controversial new Public Order Act was passed meaning protesters who interfere with “key national infrastructure”, including roads and railways, can face 12 months in jail.
The law also bans protesters from committing acts of “serious disruption” – meaning demonstrations which prevent people going about their day-to-day activities.
Wes Streeting, Labour’s shadow health secretary, told the BBC he was reassured the Met was aware of a gap in public confidence over policing and the force was “explaining and justifying” why it made some of the arrests.
He said Labour would “wait and see” whether the force got the balance right, adding accountability over policing decisions was important.