Around 7,000 protestors waved Romanian and European Union flags and chanted "party of thieves" as they called for the government to resign, with protestors joining the rally as it continued into the evening.
Thousands of people also joined demonstrations in other main Romanian cities, including Cluj in the northwest, central Sibiu and Timisoara in the west.
Video footage on social media showed police beating non-violent protesters who had been putting their hands up.
In Bucharest, some protesters attempted to force their way through security lines guarding the government building.
Centrist Romanian president Klaus Iohannis condemned the police's use of force as disproportionate. "I condemn firmly the brutal intervention of the Gendarmerie, strongly disproportional compared to the manifestations of most of the people in Victoriei Square", he wrote on his Facebook account.
An estimated 3 to 5 million Romanians are working and living overseas, the World Bank has said, or about a quarter of the European Union state's overall population.
A hundred people required medical attention after inhaling the gases, while 10 police officers were injured by hurled stones and bottles, the emergency services said. "The interior ministry must explain urgently the way it handled tonight's events".
As Romanians overseas were expected to organize a big protest against the Government on Friday, August 10, local media look into their contribution to the local economy.
Around four million Romanians work overseas from an overall population of 20 million. At least one man was detained.
Mr Ostafi said: "I left to give my children a better life, which was not possible here then".
"Unfortunately, it is still not possible, the. people who govern us are not qualified and they are corrupt", he said, adding he hoped the next parliamentary election would see a bigger turnout.
Among the crowds in Bucharest were truck driver Daniel Ostafi, 42, who moved to Italy 15 years ago in search of a future he says Romania could not offer his family, and Mihai Podut, 27, a construction worker who left in 2014, first for France and later Germany.
Peaceful protests have repeatedly been held since the Social Democrats took power in early 2017 and tried to decriminalize several corruption offences.
This year it pushed changes to the criminal code through parliament that have raised concerns from the European commission and U.S. state department.
He praised Chief Prosecutor Laura Codruta Kovesi for her efforts and said he was forced to sign the decree after being ordered to do so by the Constitutional Court.
Romania ranks as one of the EU's most corrupt states and Brussels keeps its justice system under special monitoring.