With more than 130 millimetres of precipitation already fallen in some areas of New Brunswick, forecasters predicted more localized flooding as they called for more heavy rain late Saturday.
Environment Canada said the weather system would remain potent as it moved across the southern Gulf of St. Lawrence overnight.
Canadian Hurricane Centre officials said large offshore waves of up to nine metres would contribute to heavy, pounding surf along the Atlantic coast of Nova Scotia and Newfoundland, as well as probable rip currents.
Chris Fogarty of the Canadian Hurricane Centre said the storm slowed over Fredericton and Saint John on Saturday at its peak intensity level.
Arthur was downgraded from a hurricane to a post-tropical storm Saturday morning by the time it reached the Maritimes, but it still packed a punch, causing widespread power outages in parts of Nova Scotia and New Brunswick.
Environment Canada measured wind gusts topping 116 kilometres per hour in the Halifax area.
Fogarty predicted the rain would surpass the 150-millimetre mark in Saint Stephen, N.B., on the U.S. border.
Mike Gange lives in Fredericton, one of the hardest-hit areas. He described hearing the buffeting winds tear down a maple tree in his front yard, damaging roof tiles and a rain gutter as it fell.
"(There was) a great big crack and then the whole front of the house got real dark because this 40-year-old tree split in half," said Gange.
Gange's home was not the only one in the New Brunswick capital that was damaged due to the storm.
"I drove around today and we must have seen 25 houses with big, big trees down — there were a couple of spaces where the trees are down so much that you can't go up and down the (road)," he said. "In one place you couldn't get through there if you had an army tank."
Gange said he has not seen weather this severe in his 41 years in Fredericton.
"It's like a Tasmanian devil ripping through your backyard," he said. "It's crazy here ... at times it rains so hard you can't see 10 feet in front of you."
The hurricane centre said the storm would end in the Maritimes overnight then track through the Gulf of St. Lawrence toward Newfoundland on Sunday.
Some consider the first-named storm of the hurricane season to be an early arrival in Atlantic Canada, but Fogarty didn't see that as a sign of things to come this year.
"We don't need to read anything into this storm arriving early that it will be a bad season," said Fogarty. "This could very well be the only storm we have this season, or there could be two more. We can't predict that far ahead."
Mid-afternoon Saturday, Nova Scotia Power said about 135,000 of its customers were without power.
Megan Fisher lives in Halifax's north-end and considered herself lucky after heeding advice to move her car from underneath a swaying tree.
"About two minutes after we got back from moving the car to a parking lot ... it fell right where my car was parked," said Fisher. "It would have crushed it."
Fisher described seeing the nine-metre tree lying across her driveway.
"It was just crazy," she added. "I've never seen anything like it."
New Brunswick's main electrical utility reported more than 115,000 outages by mid-afternoon. It warned some residents they could be without power for up to 48 hours because of widespread damage caused by the storm.
NB Power said the largest number of outages was in Fredericton where winds of more than 100 km/h had knocked down a number of large trees, leaving streets littered with debris.
Long lineups at gas stations and fast food restaurants led to traffic jams across the city.
City officials opened a reception and charging station at the Fredericton Convention Centre in response to the widespread power outages.
Police in Saint John said some local roads were closed because they were covered by flood water.
The storm caused flight cancellations and delays at the region's largest airport in Halifax. Other events scheduled for the weekend, including music festivals, were delayed or cancelled.
The RCMP in Prince Edward Island said a number of electrical poles had been knocked down by the storm and roads were blocked by downed trees.
Large offshore waves of up to nine metres were reported early Saturday morning off the southern coast of Nova Scotia. Closer to shore, the hurricane centre said waves of between three and five metres were recorded.
Before Arthur's arrival in Canada, it swiped the east coast of the United States on Friday but proved far less damaging than officials feared.
It left tens of thousands of people without power as it swiped at North Carolina's Outer Banks, then brought lousy Fourth of July beach weather to parts of the northeastern U.S. as it veered out to sea towards Nova Scotia and New Brunswick.
Arthur struck North Carolina as a Category 2 storm with winds of 160 km/h late Thursday, taking about five hours to move across the far eastern part of the state. At the height of the storm, more than 40,000 people lost power in the U.S.
Also on HuffPost