A 37-year-old woman was critically injured when a tree fell on her outside the General campus of the Ottawa Hospital during a violent storm Wednesday afternoon.

Paramedics said the woman suffered "multi-system trauma" and went into cardiac arrest. Staff at the hospital on Smyth Road had to lift the tree off the woman so she could be treated.

The woman is in critical condition in hospital, paramedics said.

Paramdedics also said in a tweet that a 56-year-old man was struck by a falling tree near Britannia Beach. He suffered chest injuries and is in stable condition in hospital.

The storm, which brought torrential rain and howling winds, struck shortly after 3 p.m. with surprising ferocity, catching residents across the city off guard.

<a href="https://twitter.com/BlacksWeather">@BlacksWeather</a> <a href="https://twitter.com/hashtag/Harvey?src=hash">#Harvey</a> effect near Britannia beach. <a href="https://twitter.com/hashtag/staysafe?src=hash">#staysafe</a> <a href="https://twitter.com/hashtag/ottnews?src=hash">#ottnews</a> <a href="https://twitter.com/hashtag/thunderstorm?src=hash">#thunderstorm</a> after summer like hot day. <a href="https://t.co/wLZ3Z4yeDg">pic.twitter.com/wLZ3Z4yeDg</a> —@djignesh7576

Serious wind and rain in <a href="https://twitter.com/hashtag/Centretown?src=hash">#Centretown</a> <a href="https://twitter.com/hashtag/Ottweather?src=hash">#Ottweather</a> <a href="https://twitter.com/BlacksWeather">@BlacksWeather</a> <a href="https://t.co/HXaRHdBZw7">pic.twitter.com/HXaRHdBZw7</a> —@dan613

Storm brewing in the Nations Capital <a href="https://twitter.com/hashtag/onstorm?src=hash">#onstorm</a> <a href="https://twitter.com/hashtag/ottawa?src=hash">#ottawa</a> <a href="https://twitter.com/hashtag/ottstorm?src=hash">#ottstorm</a> <a href="https://t.co/339mfFpRZC">pic.twitter.com/339mfFpRZC</a> —@Sarah_Stephen

Environment Canada had issued a severe thunderstorm warning at about 3 p.m., alerting residents that a storm south of Arnprior was moving east toward the city at 60 kilometres per hour.

It wasn't long after that residents saw just how severe it was.

Trees downed, uprooted

The storm uprooted some trees and felled others, damaging homes and cars and knocking down power lines.

Dovercourt Avenue resident Mike Greenwood said he was pulling into his driveway when the storm hit.

"I've been around for a while and I've never seen a storm like this. It was zero visibility. It wasn't even akin to a shower. It was more like a fire hose being turned on," he told CBC News via Skype.

Here's a look at storm damage Dovercourt Ave. Resident Mike Greenwood said 2 trees were struck by lighting. <a href="https://t.co/WHOz4vOrRS">https://t.co/WHOz4vOrRS</a> <a href="https://t.co/1iQH9BUFIx">pic.twitter.com/1iQH9BUFIx</a> —@CBCOttawa

He said at least two trees were struck by lightning strikes on his street.

"The lightning strikes sounded like something out of an alien attack movie. I don't think I've ever heard anything like it," he said.

"The entire thing was over in the span of about five minutes. It was one of the quickest and most violent storms I've seen in Ottawa, ever."

Heavy rain also led to flash flooding on some roads.

In addition to the woman injured at the hospital, firefighters said they were also called after a report of a collapsed scaffold at 360 Croyson Ave. Two people on the scaffold were reportedly in danger but were helped off by bystanders before firefighters arrived.

Fire crews also battled a house blaze on 2531 Regina St. after a tree fell on the house. The residents had fled the home and were not injured. The fire was under control just after 4 p.m.

Near the intersection of Carling Avenue and Maitland Avenue, winds tore part of the the roof of a low-rise apartment building off.

Hail was also reported in some parts of the city.

Hydro Ottawa reported some 8,700 customers were without power as of 6 p.m.

The storm also caused damage in neighbouring regions, including west Quebec.

The storm warning ended for downtown Ottawa shortly before 4 p.m. and for much of the region by 5 p.m.