The vigil was held in Guildhall Yard, in the heart of the City of London, to honour the dead, those injured, the emergency services and the members of the public who tackled Khan.
He also praised the bravery of the people who confronted Khan as he carried out his attack.
Merritt was reportedly a Cambridge University graduate who was working for its "Learning Together" initiative run by the Department of Criminology to promote prison-based education.
Jones's household described her as "a amusing, kind, positive influence" whose demise leaves "a huge void".
Police said Khan was convicted in 2012 of terrorism offences and released last December "on licence", which means he had to meet certain conditions or face recall to prison.
Merrit's family described him as "our lovely, talented boy" and said he "died doing what he loved".
"She was intent on living life to the full and had a wonderful thirst for knowledge, enabling her to be the best she could be", the family said.
The family said she wanted to specialise in victim support.
"Saskia had a great passion for providing invaluable support to victims of criminal injustice, which led her to the point of recently applying for the police graduate recruitment program, wishing to specialize in victim support", the family said.
"We believe that the attack began inside before he left the building and proceeded onto London Bridge", Basu said. "Saskia will leave a huge void in our lives and we would request that our privacy is fully respected", the Jones household wrote.
Johnson blamed the previous Labour government for changing the law in 2008 to allow for the early release of prisoners and vowed to introduce minimum 14-year sentences if he regains power in the December 12 vote.
"R.I.P Jack: you were a attractive spirit who always took the side of the underdog".
"We know Jack would not want this bad, isolated incident to be used as a pretext by the government for introducing even more draconian sentences on prisoners, or for detaining people in prison for longer than necessary", the statement read.
The victims were graduates of the University of Cambridge.
On that occasion, eight people were killed and 48 wounded before the three attackers, who were wearing fake suicide devices, were shot dead by police.
Regardless of Khan carrying what gave the impression to be an explosive vest that was later chose to be pretend, a pair of bystanders helped subdue him with a fireplace extinguisher and a narwhal tusk pulled from the wall of Fishmongers' Corridor.
The Dean of Southwark Cathedral, the Very Reverend Andrew Nunn, said many people were struggling with what happened. He also spoke to US President Donald Trump on Saturday, who expressed his condolences.
Officers say one of the injured has returned home while the other two remain in hospital for treatment but are in stable condition.
An urgent evaluate of terrorists launched from penal complex was as soon as launched by the Ministry of Justice following the attack on Friday.
Johnson said around 74 people convicted of serious offenses had been released under the legislation, and that they were being monitored to prevent any threat to the public.
Some people at the event, including prison staff and former prisoners, put their lives in danger to restrain the attacker until police arrived, officials said.