A mural of teenage climate activist Greta Thunberg has been defaced with pro-oil and derogatory messages days after it was created.

The vast artwork appears to depict the Swedish campaigner during her United Nations speech last month when she criticised world leaders for their “betrayal” of young people through their inertia over the climate crisis.

It was created by artist AJA Louden the Canadian city of Edmonton on Friday, hours after Ms Thunberg attended a climate rally nearby.

However, on Sunday, as a journalist was filming the portrait, a man approached the wall and began to spray paint a message across her face.

It read: “Stop the lies! This is oil country!”

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Explaining he had decided to “deal with it” as soon as he saw photos of the mural on social media, the man reportedly said: “This is Alberta. This is oil country. My father has worked in the oil industry. We don’t need foreigners coming in and telling us how to run our business, support our families, put food on our tables … just shut up until you have solutions.”

The man said his father, who had recently passed away, would have been “disgusted” by the mural.

Later that day, the Canadian Broadcasting Corporation (CBC) reported that they returned to find another man adding a French slur.

He also wrote that Ms Thunberg should leave the country. He refused to speak to reporters.

The Canadian province of Alberta, where Edmonton is situated, is well known for its oil and gas industry.

Home to the country's vast oil sands, the region holds the world’s third-largest crude reserves, but has struggled to recover from the 2014-15 global oil price crash because of delays in building new export pipelines, which have been met with strong opposition from environmental and indigenous groups, as well as regulatory hold-ups.

With historic ties to the industry, many energy sector workers and the Alberta government feel the oil sands, scorned by environmentalists for their high carbon emissions intensity, have been unfairly targeted. They say the sector is making progress cutting greenhouse gas output.

Workers’ rights groups have previously called for mass retraining in renewables to ensure former oil and gas employees are not left behind in the ongoing transition. The energy sector provides 150,000 jobs in the region.

This tension did not go unnoticed during Ms Thunberg’s appearance in Edmonton, the provincial capital.

While thousands turned out to demonstrate alongside the Swedish activist, holding banners reading “be a better ancestor”, a small group of counter-protesters led by a truck convoy of oil and gas workers also made their presence felt, their numbers reportedly bolstered by members of far-right groups.

“We’re not doing this because it’s fun or because we have a special interest in the climate or because we want to become politicians when we grow up,” Ms Thunberg said as truck horns blared in the background. “We’re doing this because our future is at stake.”

The mural’s artist told local media he had been inspired by Ms Thunberg’s speech and wished to keep the conversation going with his artwork.

He told CBC that he had not been perturbed by the initial pro-oil statement: “Nothing lasts forever — one of my favourite things about that wall is that anyone is allowed to express themselves there, so I’m not upset at all.

“I haven’t seen what went over it, but if anyone is upset about what was painted over the portrait, they can just paint back over it, it’s not a big deal at all.”