SAGUENAY, Que. -- For anyone wondering why Donald Trump has been tweeting about Canadian agriculture lately -- Justin Trudeau insists that he is the instigator.

The prime minister said Thursday that the U.S. president's complaints on Twitter about Canada's trade barriers are the result of his refusal to give in to Trump's demands to do away with the country's supply management system.

Trudeau's comments were meant to address fresh criticism from Canadian farmers who fear he is prepared to open up the system to provide more access to American competition.

The issue followed him to Saguenay, Que., where producers concerned about the future of supply management demonstrated outside one of his events.

"There's a reason why Donald Trump continues to write tweets on dairy products and Canada -- it's because I've told him many times: 'No, he won't touch, we won't touch our supply management system,' " Trudeau told reporters.

"We will always defend our supply management system because it works."

As if on cue, Trump responded on Thursday night:

"Prime Minister Trudeau is being so indignant," ..But he doesn't bring up the fact that they charge us up to 300% on dairy -- hurting our Farmers, killing our Agriculture!"

Trudeau made the remarks a day before he was scheduled to host G7 leaders, including Trump, in the Quebec town of La Malbaie.

The G7 summit is expected to produce heated discussions over trade among the leaders in the exclusive club of wealthy democracies.

In recent weeks, Trump and Trudeau have exchanged tough words over trade -- a highly unusual development in the countries' long-standing, close relationship.

The pair have sparred over Trump's move to impose hefty steel and aluminum tariffs. Canada has responded by threatening to apply tariffs of its own on the U.S.

Trudeau said they've also disagreed over Trump's call for Canada to open up its supply management system, which protects dairy, poultry and eggs farmers, during the difficult ongoing renegotiation of the North American Free Trade Agreement.

In recent days, Trump's Canada-focused tweets have included:

-- "Canada has treated our Agricultural business and Farmers very poorly for a very long period of time. Highly restrictive on Trade! They must open their markets and take down their trade barriers!"

-- "Canada has all sorts of trade barriers on our Agricultural products. Not acceptable!"

-- "Farmers have not been doing well for 15 years. Mexico, Canada, China and others have treated them unfairly. By the time I finish trade talks, that will change. Big trade barriers against U.S. farmers, and other businesses, will finally be broken. Massive trade deficits no longer!"

And following Trudeau's remarks Thursday, Trump tweeted this: "Please tell Prime Minister Trudeau and President Macron that they are charging the U.S. massive tariffs and create non-monetary barriers... Canada keeps our farmers and others out. Look forward to seeing them tomorrow."

While the steel and aluminum tariffs are expected to take centre stage at the G7 summit this weekend, the Canada-U.S. battle over supply management is likely to surface during conversations between Trump and Trudeau.

Supply management also remains a tricky domestic political issue for Trudeau.

On Thursday, dozens of noisy demonstrators waited outside in the light rain for a chance to tell Trudeau that he must fully defend supply management in the NAFTA talks. Trudeau was in the area to lend a hand to the campaign of a local Liberal candidate running in an upcoming federal byelection.

Following Trudeau's news conference, he headed outside and jumped up on the tail gate of a pickup to talk to the protesters, who held placards demanding the system be left untouched.

Trudeau held a microphone and said Trump tells him every time that he wants the supply management system eliminated.

He said he responds by telling Trump he won't sign NAFTA without supply management.

"That is a promise," Trudeau said as many farmers cheered and applauded.

Seconds later, however, shouts from the crowd demanded Trudeau to guarantee that supply management would be 100 per cent untouched under a new NAFTA.

"I guarantee you 100 per cent that I will defend supply management system," Trudeau responded as more and more members of the crowd, disappointed by his answer, started shouting over him.

"I hear you, I understand your concerns -- but I don't want to start detailing all that we are trying to do in terms of talking to the Americans. There are many issues that we will discuss."

Michel Frigon, a dairy farmer, said he was "extremely worried" that Trudeau refused to promise he would protect the supply management in its entirety. He fears Ottawa will open it up a bit like it did for other trade deals.

"I'm extremely disappointed with what we just heard," said Frigon, the regional vice-president for Quebec's milk producers' association.

"It doesn't reassure me at all... When you chip away at the foundation little by little, it will eventually fall down. That's what they're doing."