The Met Police abused anti-terror powers when it stopped and arrested a French publisher, an independent report has found.
Jonathan Hall KC said the arrest of Ernest Moret was at odds with the rights to freedom of expression and protest in a democracy.
Mr Moret, 28, was stopped in London in April over his alleged involvement in the French pension protests.
The Met has referred itself to the Independent Office of Police Conduct.
Mr Moret, who works at Paris-based publisher Editions La Fabrique, was detained at St Pancras railway station after travelling on the Eurostar to attend the London Book Fair.
He was bailed and later released under investigation. Last month, police said he would face no further action.
Mr Hall, the UK's independent reviewer of terrorism legislation, said the Met should have decided "not to exercise" Section 7 powers to carry out the arrest.
Schedule 7 powers, listed under the Terrorism Act, allow police to stop anyone at the border without any grounds for suspicion to check if they are terrorists.
"The problem with exercising counter-terrorism powers to investigate whether an individual is a peaceful protester or a violent protester is that it is using a sledge-hammer to crack a nut," the report said.
The KC wrote: "I have reached the clear conclusion that this examination should not have happened, and that additional safeguards are needed to ensure it is not repeated.
"Schedule 7 power, however useful and justified in some cases, is powerful. It must therefore be exercised with due care.
"In my view, based on the information provided, police both could have decided not to exercise the power, and should have decided not to exercise the power," he added.
Mr Hall said police wrongly told Mr Moret that he would never be able to travel internationally again if he was convicted for not sharing the PINs to his confiscated iPhone and laptop.
The warning was "exaggerated and overbearing", his report said.
The report will also recommend to the government that the code of practice for Schedule 7 is amended so it cannot be used in such circumstances in the future.
Mr Hall warned that there was a risk of it happening again if "modest" safeguards were not made.
Mr Moret's employer, La Fabrique Editions, and Verso Books - La Fabrique Editions' British sister publisher - condemned the arrest at the time, describing the detention as an "outrageous and unjustifiable infringement".
They said officers told Mr Moret, who works as a foreign rights manager, he had taken part in demonstrations about President Emmanuel Macron raising the retirement age from 62 to 64 in France.
Pamela Morton, senior books and magazine organiser at the National Union of Journalists, said it "seems extraordinary" that British police used terror legislation to arrest a publisher "who was on legitimate business here".
Cdr Dominic Murphy, who leads the Met's counter-terrorism command, said: "Schedule 7 is an important power in protecting the borders of the UK and remains a vital tool in our efforts to counter the terrorist threat and keep the public safe.
"But the public rightly expects that the use of such powers is always carefully considered and, as Jonathan Hall KC states, that there is constant vigilance and attention to safeguards to ensure it is not used in a way that is contrary to individual rights and the wider public interest.
"We will now take time to fully review the report's finding and its recommendation in relation to further amending the code of practice and we will also look to consult with our operational partners on this."