Prime Minister Malcolm Turnbull has asked Australians to pray for seven-year-old Julian Cadman, who has been missing in Barcelona since the terror attacks.
Mr Turnbull said the "shocking terrorist attack" had resulted in Australians being injured and grave fears were held for Julian, whose mother remains seriously injured in hospital.
"There's a little Australian boy whose mother was badly injured and he is lost, he's missing in Barcelona," Mr Turnbull said.
"So I think we should all in our private moments say a prayer for that little boy. All of us as parents know the anguish his father is going through and his family is going through as they rush to find him in Barcelona."
Julian's father, Andrew Cadman, is on his way to Spain.
Mr Turnbull said "we stand in resolute solidarity with the people of Spain in the battle to defeat these Islamist terrorists".
"We will never bow, we will never be cowered by them, we will defeat them and destroy their evil ideology."
Family and friends of Julian are sharing photographs of the little boy widely across social media in the hope he will be found alive.
At least 13 people were killed and more than 100 injured in the terror attack on Barcelona's Las Ramblas on Thursday.
Jom Cadman, Julian's mother, was found in hospital and is in a serious but stable condition. Her son has yet to be found.
The mother and son were visiting Barcelona for a wedding this weekend when they were caught up in the deadly terror attack.
Mr Cadman stayed behind in Australia.
A longtime friend of the family, known only as Scott, told Sydney's 2GB Radio that Mr Cadman was flying to Spain on Friday evening to be reunited with his seriously injured wife and begin the gut-wrenching search for his missing son.
Mr Cadman heard about the news of the terrorist attack when he arrived at work just before 6am on Friday morning.
He began frantically calling his wife, but couldn't get a hold of her.
"He turned up to work this morning, turned the news on at 6 o'clock, heard about it, tried to ring his missus but couldn't get a hold of her," Scott told 2GB's Ben Fordham on Friday afternoon.
"By about 8.30am we found out that it was her in the hospital. She's in a very serious condition... But we found out about an hour later that his seven-year-old son was with her, he's missing and we haven't heard anything since. They don't know if he's still with us or not."
Scott said Mr Cadman was "absolutely beside himself".
"He can't do much more than sort of put one foot in front of another at the moment," he said. "I need to know something, I can't believe 10 hours can pass and they can't link a mother and a son together. He's obviously somewhere, we just need to know where."
Julian's cousin, George Cadman, wrote on Facebook: "My cousin, Julian Alessandro Cadman is missing. Please like and share.
"We have found Jom (his mum, my cousin-in-law) and she is in a serious but stable condition in hospital.
"Julian is 7 years old and was out with Jom when they were separated, due to the recent terrorist activity.
"Please share, especially if you have family or friends in Barcelona. Thank you."
The boy's aunt, Claire O'Sullivan, and his grandfather Tony Cadman, from Sydney, also posted similar pleas.
His distraught aunt Maricar Querimit Estera said his "family were waiting for him."
"My nephew is still missing ... they need help to find him quickly ... pls share," she posted on Facebook.
Another family friend, Sarah Auzner, pleaded with the public to share his photo.
"Please, please share his picture so he can be with his family," Ms Auzner wrote.
"We are beside ourselves with worry. Andrew and Jom Cadman we are praying for you."
Foreign Affairs Minister Julie Bishop confirmed that four Australians had been hurt in the attack, while one person is missing.
The Department of Foreign Affairs and Trade has been contacted for comment.
Sydney Morning Herald, with Melissa Cunningham, Liam Mannix, Telegraph London