Unions in the northern Italian city initially lobbied to have the vessel banned, but their efforts failed and the Bahri-Yanbu docked there earlier this week.
It was met by protesters. One carried a sign which read “no to war”.
Union workers then refused to load the generators.
“We will not be complicit in what is happening in Yemen,” the group’s leaders said in a statement.
The workers said that although the generators were marked for civilian use, there was a risk they would instead be redirected to Yemen.
Port officials said non-critical goods were loaded but the generators were left on the quay.
At least 6,872 people are believed to have died in the Saudi-led war in Yemen, which began in 2015. A further 10,768 civilians have been wounded. Most of the casualties were killed or injured in Saudi-led airstrikes, prompting international condemnation.
The war has also left much of the surviving population on the brink of famine.
The vessel left without a cargo of weapons on 9 May, after a rights group ACAT tried to block the cargo on humanitarian grounds.
The group argued in a legal challenge that the consignment should be halted, as it contravened a United Nations treaty.
Although a judge threw out the case the ship left shortly afterwards without the cargo.
Additional reporting by agencies
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