Twenty doves were released during the civic event to commemorate the Warrington bombing on 20 March 1993, which instantly killed three-year-old Jonathan Ball.
Tim Parry, aged 12, was the second victim when he died five days later.
A further 56 people were injured in the blast at Cheshire town's shopping centre, from bombs placed in litter bins.
No warning was given and nobody has been prosecuted for the attack.
Addressing the crowd on Saturday, Tim's father, Colin, said what began as an act of terrorism led to a change in attitudes on both sides of the divide.
Mr Parry and his wife set up the Tim Parry Jonathan Ball Foundation for Peace in the aftermath, which he said has helped build bridges between divided communities by working towards peace and reconciliation.
The Mayor of Lisburn, Alderman William Leathem, also attended the event.
He said he was there to pay tribute to "the people of Warrington for their determination and resolve over the last 20 years."
A further minute's silence is to be held on Wednesday at 12.27pm - exactly 20 years on since the bombing.