Charity worker Nazanin Zaghari-Ratcliffe, from Hampstead, was arrested at Tehran Airport on April 3 last year while visiting family with daughter Gabriella.
She was imprisoned for five years in September for allegedly plotting to topple the Iranian government and lost an appeal against her sentence in January but maintains her innocence.
On Sunday - the 365th day since the arrest - family and friends gathered at Fortune Green close to Mrs Zaghari-Ratcliffe's home in north-west London.
Supporters tied yellow ribbons to a tree in the park along with quotes from inmates at Evin prison in Iran, where Mrs Zaghari-Ratcliffe is being held, describing what they would do with one day of freedom.
Mrs Zaghari-Ratcliffe's wish reads: "My fondest dream has always been to arrive at our home, you ask me if I want to have a cup of tea, then make me one.
"I just sit back and watch you two play. This is the image I had most when in solitary confinement.
"How I wish I could watch you both dance in the middle of our sitting room to the Michael Jackson music - like when Gabriella was only tiny."
She also told how she would like to "put a huge paper on the wall" and "draw a world in which there are no prisons, walls or fences - and let Gabriella do the colouring".