Justin Trudeau has apologized for a sarcastic response to indigenous activists protesting against the contamination of drinking water, admitting on Thursday that he handled the situation poorly.

“From time to time I’m in situations where people are expressing concerns … I always try to be respectful [but] I didn’t do that last night. I lacked respect towards them and I apologize for that,” the Canadian prime minister told reporters.

Trudeau was widely criticised for his reaction when the woman interrupted his speech at a party fundraising event to demand compensation for an indigenous community that has suffered mercury poisoning for more than 50 years.

“Prime Minister Trudeau, people in Grassy Narrows are suffering from mercury poisoning,” said the woman, who also attempted to unfurl a banner in front of the podium. “You committed to addressing this crisis,” she shouted as she was pushed out by event security.

Trudeau responded by saying: “Thank you very much for your donation tonight. I really appreciate it,” prompting cheers and laughter from the donors in attendance.

Fellow parliamentarians quickly took him to task over social media.

“Just another telling episode about how he really doesn’t … care about Indigenous Rights and people. And the Liberals in the room found his ‘thank you for your contribution’ funny? It isn’t arrogance or a renewed relationship, this is a huge ‘We Lied To You Again!’,” tweeted MP Romeo Saganash.

Saganash, a member of the leftwing New Democratic party, has been a vocal critic of Trudeau, saying last year that he “doesn’t give a fuck” about the rights of indigenous people.

Charlie Angus, another NDP member, described Trudeau as a “smug, mean, aloof ass”.

“When #grassynarrows asked @JustinTrudeau to see for himself the poisoning of the people he refused. He sneered at them for crashing his $1,500 fundraiser for liberal elite “thank you for your donation,” he tweeted.

Trudeau told reporters on Thursday morning that he would ensure the protesters – who paid $1,500 to attend the event – would receive a refund.

Conservatives – whose prospects in the fall general election have improved in recent weeks – also took aim at Trudeau.

“We all have a long way to go in building meaningful relationships with First Nations; reconciliation, and more. This is not a step in that direction. Also – this is what I look at and deal with every day in the House; the smug grin, the arrogant laughs, the dismissiveness,” tweeted Conservative parliamentarian Michelle Rempel.

The incident comes amid more dismal polling for Trudeau, which show support for him continues to fall: Trudeau’s approval levels are now below those of the US president, Donald Trump, according to polling data from Ipsos Reid.

The erosion of support for Trudeau is largely attributed to a continuing scandal in which the prime minister and his staff are accused of pressuring the former attorney general to abandon the prosecution of engineering company accused of committing fraud and bribery.