Lisbon's City Hall has been fined €1.25m ($1.4m, £1m) for passing protesters' details to the foreign embassies they had been picketing.
Portugal's data commissioner said the mayor's office had committed 225 breaches of demonstrators' personal data between 2018 and 2021.
Their details were shared with the the embassies of several countries, including Russia's, by former Socialist Mayor Fernando Medina's office.
He has apologised for the breaches.
Several of the violations occurred just months before the scandal became public, when some of the protesters raised their concerns with the city council, the data commissioner said.
The mayor's office is said to have passed on information about the organisers of 52 protests to embassies, including Russia's, Cuba's, and Israel's.
More than 100 other breaches that occurred since 2012 were not covered as they pre-dated the introduction of the European Union's General Data Protection Regulation (GDPR) - which bans such data sharing - in May 2018.
The report added that some of the breaches could have attracted fines of up to €20m ($23m, £16m) each, but said that the commissioner had refrained from imposing these due to the effect of the pandemic on public finances.
In a statement issued on Friday, the mayor's office, now headed by Social Democrat Carlos Moedas, said the decision was a "heavy legacy the previous leadership... left to the people of Lisbon".
"We will evaluate this fine in detail and how best to protect the interests of citizens and the institution," the statement added.
One of those impacted by the breaches is Ksenia Ashrafullina, a Russian-Portuguese organiser of a rally outside the Russian embassy in support of jailed Kremlin critic, Alexei Navalny.
She welcomed the data commissioner's decision, but warned that the leaks could have lasting effects on those involved.
"I'm worried about what would happen if I ever needed to go back to Russia," she told the Reuters news agency.
The revelation of the data breaches caused significant controversy last summer, and is believed to have been a major contributing factor to Mayor Medina's defeat in last year's elections.