"We will not be complicit in what is happening in Yemen," union leaders said in a statement.
Italian unions refused Monday to load electricity generators onto a Saudi Arabian ship with weapons on board in a protest against the war in Yemen.
The Bahri-Yanbu vessel loaded arms in the Belgian city of Antwerp earlier this month, but was prevented from picking up another consignment of weapons in the French port of Le Havre following protests by humanitarian groups.
Rights campaigners say the weapons contravene a U.N. treaty because they might be used against civilians in Yemen, where a Saudi-led military coalition is battling the Iran-backed Houthis in a war that has killed thousands.
Unions in Genoa had tried to have the boat banned from Italy, but the ship docked just after dawn, met by a handful of protesters who gathered on the quay.
"No to war" read one of their banners.
Union workers refused to load two generators aboard the boat, saying that although they were registered for civilian use, they could be instead directed to the Yemen war effort.
"We will not be complicit in what is happening in Yemen," union leaders said in a statement. Port officials confirmed the generators were blocked on the quay, but said non-critical goods would be loaded.
The four-year conflict in Yemen has devastated the country, leaving much of the population on the brink of famine.
The vessel was expected to leave Genoa for Jeddah, in Saudi Arabia, later in the day.