Animal rights activists have been out on the ice, filming Canada's largest seal cull for more than 50 years.
Previous seal hunts have provoked international outrage
The government is allowing more than 300,000 seals to be killed, arguing that the campaign is both ecologically sound and economically justified.
Protests helped end the hunting of young seals for their pelts off Canada's east coast 25 years ago.
"We think it is inherently inhumane," said a spokesman for Ifaw, the group which is monitoring the cull.
Sealers are out on the ice floes and islands off the coast of Newfoundland and Labrador for the annual hunt.
Under new guidelines, most seals are meant to be shot and not clubbed to death in a bid to make the killing more humane.
Around 140,000 seals are expected to be slaughtered by the end of Tuesday.
The BBC's Lee Carter in Toronto says the huge international campaign against the hunt seems to have caught the Canadians by surprise.
One campaigning group, the US-based Humane Society, has been taking full-page adverts in prominent US newspapers to urge a travel boycott against Canada.
Groups accuse officials of "attempting to scapegoat seals" for their own fisheries mismanagement.
Canada argues that the seal population is exploding
The International Fund for Animal Welfare (Ifaw) says government guidelines on humane hunting methods are being ignored.
"To call it a hunt is a misnomer," spokesman Chris Cutter told BBC News Online.
"These are baby animals that haven't learnt to swim. It's like picking blueberries," he said.
"The major aim is to preserve pelts, not to kill humanely."
However, some of the major activist groups that targeted sealing in the past said they have other issues to address this year.
Greenpeace Canada is "not actively campaigning" against the hunt and is focusing on issues such as genetically modified foods and climate change, a spokesman said.
The seal hunt in Newfoundland and Labrador went into decline 25 years ago, after images of hunters clubbing infant seals horrified TV viewers across the world.
The US banned imports of seal products in 1972 and the EU followed suit a decade later, with a ban on white pelt imports taken from the youngest babies.
Up to 350,000 baby harp seals to be killed this season
Preliminary culls started at the beginning of April
2,500 men and 150 trawlers to gather for intensive 36-hour phase of cull on 12 April
Up to 10,000 seals to be killed per daylight hour
Killing of "whitecoat" seals - aged up to 12 days - banned
As a result, the Canadian government reduced quotas for seal hunting to as low as 15,000 annually - mainly for meat and local handicraft. But with fur again in fashion, the hunt is back.
Canada increased the quotas last year, allowing a million seals to be killed over the space of three years.
Canadian Natural Resources Minister John Efford said many claims about the hunt were simply wrong.
He argued that the seal population was exploding - with an estimated 5.2 million harp seals in the North Atlantic at present - and commercial fish stocks were vanishing.
Mr Efford added that the cull was important for the local economy during a traditionally slow time of the year.