document: Richard Ratcliffe will celebrate Gabriella's second birthday near the Iranian embassy in London's Hyde Park.
  His wife Nazanin Zaghari-Ratcliffe, 37, was arrested on 3 April at an airport in Iran after visiting her family.
  He says she has been held without charge, while Gabriella is in Iran because her passport has been taken.
  Mr Ratcliffe, who is from north London and has been advised to stay away from Iran, says his British-Iranian wife has been held in solitary confinement over an issue of "national security".
  Their daughter, a British citizen, has had to stay with her grandparents in Iran because she cannot return to the UK without her passport. Under Iranian law only her father or mother can bring the toddler home.
  More than 757,000 people have signed a petition calling for Prime Minister David Cameron to intervene in the case.
  The Free Nazanin Campaign event will also be the culmination of its birthday card campaign, which has seen thousands of people sending cards for Gabriella via Iranian embassies around the world.
  It will be attended by family and friends, neighbours, Gabriella's MP Tulip Siddiq and representatives of campaign website Change.org.
  Mr Ratcliffe will attempt to present a card to the embassy at 13:00 BST and ask that they pass it on to his daughter.
  He told the Press Association news agency that he was "overwhelmed" by the public outcry about his family's situation.
  Of his daughter, he said: "I am grateful that she is with her loving Iranian grandparents but, nonetheless, I want her and her mother together and back home with me as soon as possible.
  "It has been really moving for all my family here to see the kindness of people across the country connecting with our story, making cards for Gabriella with their own children or grandchildren, and doing something to make Gabriella's world a better place.
  "In my bones, I feel it is that kindness that will in the end bring them home."
  Mr Ratcliffe says his wife had been about to return to the UK when she was detained by the Iranian Revolutionary Guard and taken to an unknown location in Kerman Province, 621 miles (1,000km) south of Tehran.
  He says Mrs Zaghari-Ratcliffe, who works as a project coordinator for the Thomson Reuters Foundation, has not been allowed access to a lawyer or to see her daughter.
  According to him, she has not been charged but has been forced to sign a confession "under duress", despite not knowing what she was confessing to.
  Mr Ratcliffe has previously told the BBC there was nothing in her work or personal background to explain why she was detained, saying she had travelled to the country before without any problem.
  He said her family in Iran heard nothing after her arrest for three days, when she was allowed to phone and tell them she was safe.
  They were told she would be released within a couple of days, he said, but after they heard nothing they later learned she had been transferred to southern Iran.
  Mr Ratcliffe has said he was going against the advice of the Foreign Office in speaking publicly about his wife's detainment because he hoped public pressure might help secure her release.
  A Foreign Office spokeswoman said: "We have been providing support to the family of a British-Iranian national since we were first informed of her arrest, and will continue to do so."
summary: The father of a toddler stuck in Iran without either of her parents nearby plans to sing Happy Birthday to her via Skype at a campaign event later.