Andrzej Duda took part in ceremonies held in Gdynia, at a monument to victims, close to the railway station where shots were first fired at workers on December 17, 1970.
"In this place, exactly 50 years ago, workers were going to their morning shifts in the shipyard, having been called to work,” the president said in an address on Thursday morning.
"In this place, on this bridge at the Gdynia Stocznia railway station, armed men were waiting for them," he continued. "Soldiers and police unexpectedly (...) opened fire on them. It was one of the most symbolic events of the entire period of communist rule in Poland after 1945."
Protesters had clashed with communist security forces in Gdansk, Gdynia, Szczecin and Elblag after the government raised the prices of basic foodstuffs. Official figures put the number of deaths at 45, and 1,165 people with injuries, but some historians say the real numbers were higher.
December 17, the bloodiest day, became known as “Black Thursday”.
"These people, workers, shipyard workers, were not taking part in a demonstration, they did not come here to oppose anything. They were just going to do their everyday jobs, to earn a living for their families. And they died," the president said.
Duda added that "this was the end of any illusions about what communist rule was and who represented it."
"It was a collapse," he said. "The communists themselves violated the values on which they were founded, an attempt to create an atmosphere of some understanding between the government, which called itself the people's government, and workers. If the government shoots at workers who are going to work, then it's the end. And that's what happened."
The president reminded his audience that the workers' complaints were not political but economic. "It was all about price increases, as people's living standards deteriorated, but from this moment their grievances became political and this is how Solidarity (trade union - PAP) was born in 1980."
The president said that "the free Poland that we enjoy today was born from the blood shed in subsequent Decembers, in 1970 and later in 1981."
After the Gdynia ceremonies, the president went to Gdansk, where he lit a candle at the Monument to the Fallen Shipyard Workers of 1970.
The tragedy of Black Thursday became immortalised by the iconic photograph of a procession of workers carrying the body of 18-year-old Zbigniew Godlewski on a door, with a blood-stained Polish national flag.
He had been killed soon after arriving to work by train, near the Gdynia Stocznia station. (PAP)