In fact, the stunned bride only found out she would be the star attraction at the wedding when the doors of the church were opened.
Her husband, Gustavo, managed to arrange the ceremony at the church in Sarandi, Buenos Aires, Argentina, without his wife suspecting a thing.
And her incredible reaction was caught on camera as dozens of her friends helped surprise her.
An almost nine-minute long film of her entrance, caught by a friend, shows how staff at the church in the city opened the massive wooden doors to the church and people start applauding.
Mercedes appears momentarily stunned before it appears someone tells her she has, in fact, arrived at her own wedding.
She had been told she was going to a friend's ceremony.
But she then breaks down into tears as she realises what her soon-to-be husband has done.
Every time she composes herself, as her friends manage to put a veil on her head, she breaks down into tears as she spots another old friend.
At one stage, she spots what are thought to be her three children with Gustavo; Gaspar, 13, Gianni, 10, and four-year-old Gala.
It takes her a number of minutes to make her way down the aisle with her father as she stops to greet guests.
But she eventually gets to the altar where she gives Gustavo a kiss.
Later that night, while the couple danced, she told him: "I can't believe you did this, you're crazy."
The childhood sweethearts have been together for 30 years before Gustavo made Mercedes an honest woman, according to Argentinian site tn.com .
He is said to have come up with the idea five years ago and spoke to a father at the Sacred Heart of Barracas Church who agreed to the plan.