London: "Please help, I just want to find her, she is an amazing person and it's just not fair that something bad might have happened to her. I wasn't able to help her and it's killing me."

Pri Gonçalves, 26, is desperately searching London for her friend - Australian au pair Sara Zelenak - after the 21-year-old went missing on Saturday night.

"She is an amazing girl," Pri said. "We were having fun that day, she has plans and all I want is for her to achieve them all."

Canadian woman Christine Archibald is the first of the seven people killed in Saturday night's terror attack to be identified. Others who were killed are also thought to be from abroad. A further 36 people are injured, 18 in a critical condition, after three terrorists mowed down pedestrians and launched a frenzied stabbing attack on people dining in the city's popular Borough Market.

Pri and Sara were just leaving London Grind, a hip eatery and bar on London Bridge, when they heard the rented white Renault van crash.

"We both got curious and went back a few steps to see what had happened when people started running and shouting 'run'. At that moment I got scared and desperate so I ran," Pri said.

"Sara was with me when people started running and shouting but at that very moment we got separated."

Sara was wearing high heels and Pri fears that may have caused their separation. "I ran thinking she would be running with me but I looked back and she wasn't there."

Pri told Fairfax Media that witnessing the attack was the scariest moment of her life, made much more distressing by getting separated from Sara, who hasn't answered her phone or checked her WhatsApp since 10:03pm on Saturday night

"I thought I was going to die, it was by far the most traumatising, shocking and scariest moment of my life," she said.

"I'm feeling terrible and scared of crowded places, [of] people and of the tube. I can't be alone and what makes this worse is to know that Sara was with me when [she went] missing.

"I just want to find her. I'm doing whatever might help people find her and I just want people to help and be able to find her."

Pri and Sara met on the train before Easter. Both were au pairs at Herne Hill in south London and struck up a bond immediately. On Saturday night they were celebrating Sara's new job in Westminster. They met at London Bridge station and went for a walk along the river. After finishing dinner and a drink at Grind, Pri suggested they go and look for a bar in Borough Market. 

But when a white van began crashing into people, she ended up running through the market, to screams as the stabbing attacks began.

" When I was close to Borough Market I saw a guy on top of another guy who was screaming and then I knew something really bad was happening. I think he was stabbing him," Pri said. "So I started calling her but she never picked up the phone." 

Pri's first call to Sara was at 10.09, one minute after police received their first report of an incident on the bridge. She tried again two minutes later and again at 10:16 and 10:17 and then another six minutes later. There is still no answer.

Pri hid in the bathroom of a nearby restaurant with another woman as she heard the screams outside. Only later did she realise she had also heard gunshots. 

"Then there was silence and then police arrived, they shouted at us asking us to run," she said.

Police fired 50 rounds to shoot dead the three attackers who were wearing canisters - or hoax suicide vests. A member of the public was caught in the gunfire.

Pri says she is wracked by guilt at the disappearance of her friend. "I wish I could have grabbed her hand."

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with Emily Woods