Two hostages and their radical Islamic kidnapper were killed Monday after a 17-hour siege that ended when Australian police stormed the cafe in Sydney where the victims and 15 others were held captive.

Heavily armed with rifles and flash grenades, cops in full combat gear blitzed the Lindt Chocolat Cafe in Sydney’s central business district — finally ending the marathon drama that began when a militant Muslim preacher began terrorizing his hostages with a sawed-off shotgun and threats of planted bombs.

The stricken hostages started running from the shop with their hands in the air just before helmeted police pitched the flash grenades into the cafe, ending the bloody standoff that kept the city on edge for nearly a day.

One of the dead hostages was identified as mom-of three Katrina Dawson, the 38-year-old sister of well-known Sydney lawyer Sandy Dawson, according to the Daily Telegraph. She also was a lawyer and worked across the street from the cafe.

The other hostage killed was 34-year-old shop manager Tori Johnson.

Three more people were seriously injured, including a cop who was grazed in the face by a bullet.

Police had been negotiating with Monis for a large part of Monday before officers, under the cover of 2 a.m. darkness, stormed the cafe after hearing gunfire inside.

It wasn’t clear whether the dead and injured were fired at by Iranian-born gunman Man Haron Monis or struck by police bullets.

No bombs were found.

Monis, 50, was a onetime political refugee who was facing charges including sexual assault and accessory to murder in separate cases.

Last year, he was sentenced to 300 hours of community service for writing offensive letters to families of soldiers killed in Afghanistan — and had just lost an appeal in the case Friday.

During the standoff, Monis forced at least three female hostages to appear on videos outlining his demands, which included an ISIS flag and a phone call from the prime minister.

“We don’t understand why these demands haven’t been met yet,” one shaken woman recited into the camera. “They’re not unreasonable. He’s only asking for a flag and a phone call.”

Hours later, the same woman was seen being carried out by rescuers after the siege was ended.

Earlier, Monis forced some of the hostages at gunpoint to display an Islamic flag belonging to a Syrian terror group in a storefront window.

In September, Islamic State spokesman Abu Mohammed al-Adnani issued an audio message urging so-called “lone wolf” attacks abroad, specifically mentioning Australia.

But no direct link had been made between ISIS and Monis.

“This is a one-off random individual. It’s not a concerted terrorism event or act,’’ his lawy,er Manny Conditsis, insisted to the Australian Broadcasting Corporation.

“It’s a damaged-goods individual who’s done something outrageous,’’ Conditsis said. “His ideology is just so strong and so powerful that it clouds his vision for common sense and objectiveness.”

In New York City, officials were beefing up security in the wake of the incident.

Mayor de Blasio said the city is ramping up its security at key Australian government sites, including its consul general offices on East 72nd Street.

“We are taking appropriate precautions, making sure we are beefing up our presence at locations related to the Australian government and other pertinent locations around the city,’’ the mayor told reporters.

”You’ll see substantial activity to make sure that those locations are protected. The NYPD as usual is in close coordination with our federal partners.”

The Sydney tragedy brought out the best and worst of people on the Internet.

A few people took smiling selfies of themselves standing near the scene.

But a massive Twitter campaign also popped to protect Australia’s Muslim population against potential backlash.

A Sydney TV editor created the #illridewithyou hashtag after reading a moving Facebook posting from a Sydney woman who was riding the subway Sunday and saw a female Muslim rider remove her hijab out of fear.

“I ran after her at the train station. I said ‘put it back on. I’ll walk with u’. She started to cry and hugged me for about a minute – then walked off alone,’’ the Sydney woman wrote.

The TV editor then posted Monday afternoon, “If you [regularly] take the #373 bus b/w Coogee/MartinPl, wear religious attire & don’t feel safe alone: I’ll ride with you.”

She added the hashtag, which quickly went viral.

The Lindt Chocolate companies issued a statement thanking the public for its support.

“We are deeply concerned over this serious incident and our thoughts and prayers are with the staff and customers involved and all their friends and families, the statement said. “The matter is being dealt with by the authorities and we are waiting for any updates from them.”

With Post Wire Services