Brendan Cox was telling crowds how the killed Labour MP “lived for her beliefs” and would have been spending the day campaigning for Britain to remain in the European Union.
Thousands of people were gathering at events in London, across the UK and around the world to pay tribute to Ms Cox's life and legacy on what would have been her 42nd birthday.
Many of those rallying in Trafalgar Square condemned the appearance of the pro-Brexit plane as “disgusting”, saying the banner flew over the rally several times as the memorial continued.
One person called the appearance “unbelievably tasteless”, while another said it was “completely disrespectful”.
Stella Creasy, the Labour MP for Walthamstow, wrote a tweet to the official Vote Leave campaign saying: “Flying your plane over the memorial tribute to Jo Cox in Trafalgar Square is beyond low. Have some self respect and disappear!”
She added: “It's been over twice now - that isn't coincidence @voteleave , that's just sickening disrespect!”
Vote Leave said it did not organise the plane, which had been dispatched by British aircraft manufacturer Britten-Norman as part of its own pro-Brexit campaign.
In a press release announcing the stunt, the firm said its banner would be flown around the UK between 9 June and the day before the referendum from a turboprop Islander aircraft.
"We understand that, during a pre-planned photo opportunity over Westminster today, we were visible from Trafalgar Square and that our flight coincided with an important memorial service,” a spokesperson said.
"The timing of our flight was determined by weather, air traffic control and the prior approval of the the Diplomatic Protection Group and required the consent of the Metropolitan Police.
"We are clearly very disappointed that the timing of the service and our flight have overlapped but there was no NOTAM (Notice to Airmen) requesting the avoidance and the coincidence of timing could not have been known by the crew flying the sortie.
"Our sincere apologies to anyone present who has been offended by the overflight. We would also like to take the opportunity to advise that the flight was a private one and was not sanctioned by the official Vote Leave campaign.”
The plane's engines could be heard as Mr Cox fought against tears in an emotional tribute to his late wife.
“Your support and your love has helped us all,” he told the crowds. “Today would have been Jo’s 42nd birthday and she would have spent it dashing around the streets of her hometown trying to convince people that Britain is stronger in Europe.”
He said the Labour MP would have been “amazed, baffled and humbled” at the global reaction to her death and the “outpouring of love from around the world”.
A minute’s silence was held during the hour-long celebration of her life, which was also attended by her three-year-old daughter Lejla and son Cuillin, five.
Malala Yousafzai, the education campaigner who was shot in the head by the Taliban for her activism, was among the guest speakers at the rally.
Events were being held in a variety of locations in tribute to Ms Cox's “love, energy, passion, flair, Yorkshire heritage and belief in the humanity of every person in every place”.
Bill Nighy was due to give a reading, while U2 had recorded a musical tribute in Los Angeles.
The band that played at the couple's wedding, Diddley Dee, also performed and a group of the MP's friends will form an honour guard dressed in suffragette-style sashes.
Leaders of multiple faiths were to lay 42 white roses, the symbol of Yorkshire, to mark her birthday at the event, hosted by Mrs Cox's friend, Mariella Frostrup.
International events were also taking place in Beirut, Brussels, Melbourne, Nairobi, New York and Washington DC.
Glastonbury's Park Stage will be the location of one tribute, while a charity album of music recorded by bands including Coldplay and Muse at the festival this weekend will be released in her honour.
It came after a fundraising page in Mrs Cox's memory raised more than £1 million for three of her favourite charities in just three days.
Tommy Mair, 52, has been charged with murdering Mrs Cox in a shooting and stabbing attack in Birstall on Thursday, and remains in custody.
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