Six Greenpeace activists locked themselves to a truck to block access to the Mars Petcare factory in Castlecliff for 11 cold, windy hours yesterday.
Campaigner Kate Simcock said the "passionate activists" were chained to the truck to stop other trucks coming through.
"We believe that Mars relies on a constant flow of tuna and other raw products," Ms Simcock said.
The group started the protest at 8.30am and ended it at 7.30 last night.
Whanganui residents Rae and Lanice Ranginui, who have friends and family employed at the Bryce St factory, joined the protesters in support.
Ms Simcock said the protest was against tuna from Thai Union being used in cat-food pouches and that Mars had confirmed it sourced tuna from that company.
She said Thai Union used really aggressive and destructive fishing practices and its boats had been linked to slavery scandals.
Ms Simcock said the activists would remain in place "until we're really sure Mars has got the message".
"We want Mars to urgently acknowledge the problems in its supply chain. They need to tell us what plan they're putting in place."
It was easy to supply the activists with food and drink. Toileting was more difficult and she would not say how that was managed.
"We don't really talk about it, because we respect these guys. They're doing a hard job for important reasons."
She presented a letter to Mars Petcare staff yesterday morning outlining the reasons for the protest.
A support group of about 18 hung a banner saying "Stop bad tuna" from the paws of two cats on a billboard. They added Greenpeace signs to two other cats painted on the factory, saying they were making the cats into "cativists".
Greenpeace is campaigning against the large fishing company globally. Ms Simcock said there was a meeting between Mars and Thai Union in Britain yesterday at 8pm New Zealand time. It was hoped the action influenced the meeting.
At the same time, a Greenpeace ship is pulling up Thai Union's "destructive" fishing gear.
The Greenpeace truck was blocking access to the factory, which needed a constant supply of raw materials for its 24-hour manufacturing of pet food.
The tuna supplied by Thai Union was in foil pouches of Whiskas cat food, Ms Simcock said. Greenpeace's campaign was aimed at Thai Union, but it also wanted Mars to acknowledge problems with its supply chain.
Police were aware of the protest and visited the site several times during the day, but no action was taken. A police spokesperson said they were monitoring it.
Greenpeace has been working on its Thai Union campaign for many months. The human rights actions of the company were highlighted in a New York Times article, published in July last year. It followed the case of a fisherman supplying the company.
Staff were trafficked, Ms Simcock said, their labour was indentured and they were deprived of sleep and unable to get off boats for three years. She said the company's fishing methods were destructive, had by-catch of sharks, rays and turtles, and trashed the ocean.
Thai Union is a big, Thailand-based tuna company. Its brands include Chicken of the Sea and John West.
The plant manager for Mars Petcare in Whanganui, Derek Pickering, said the company was aware of changes needed to the Thai fishing industry and was working with a third party to make improvements in that country.
He said Mars Petcare had been talking to Greenpeace prior to yesterday's actions and didn't understand the protest.
Mr Pickering said Mars Petcare "does not tolerate forced labour in any aspect of our supply chain, and we are extremely concerned about allegations of abuses taking place in the Thai fishing industry".
"We believe that, as a global business, we have a responsibility to contribute to the change that the Thai fishing industry needs to make to become a reliable and sustainable supply chain.
"As such, it is vital that we engage with, rather than abandon, the industry to improve conditions and to stamp out any human rights abuses.
"We do not believe targeting one supplier is driving the change needed in the region. We are in conversation with Greenpeace and they are aware of our plans and therefore don't understand their actions taken today."
Mr Pickering said Mars Petcare and the "reputable third party organisation" would work together "to establish transparency and visibility across all the tiers of our supply chain within Thailand".
Those findings would be ready in July and the report would be made public.
The company employs 179 people in Whanganui and is a 24-hour operation.
Mars New Zealand corporate affairs manager Victoria Hamilton said the plant operated as normal yesterday, "and we are conducting business as usual".
She said the Whanganui operation had been supplying pet-food pouch products since 2000.
"Globally, Mars purchases very small amounts of fish ingredients from Thailand. Our spend represents about 0.008 per cent of the Thai fish industry.
"In terms of Thai Union, a very small amount of tuna is obtained from Thai Union directly." She said Thai Union "has been audited like all of our tier-one suppliers".
Mars Petcare is part of the Mars corporation based in Virginia in the United States. It has annual sales of $US33 billion ($49 billion) and makes pet food and many other food products. It is owned by the Mars family. They have many factories and 75,000 staff worldwide.
The Whanganui factory was set up in Castlecliff in 1993, and $13.4 million worth of improvements, including automation, were added in 2010-11. Its product is sold in Australia and New Zealand.