A 75-year-old man hit by Masood's marauding vehicle on Westminster Bridge died on Thursday evening and was identified as Leslie Rhodes.
"Those evil and twisted individuals who tried to destroy our shared way of life will never succeed and we condemn them", Mayor Sadiq Khan said in an address to the crowd.
"After watching the awful attack in Westminster on Wednesday, Wales want to show we stand united", said Joe Redmond, joint secretary of the Stand Up For Racism Cardiff branch.
Driving over the bridge, the attacker - Khalid Masood - left a devastating trail of death and carnage in his wake, as those lucky enough to escape with their lives could only watch in horror before rushing to help those who had been injured. His wife was seriously injured and remains in hospital.
Aysha Frade, a 43-year-old mother-of-two, was killed by Masoon on Westminster Bridge where she was travelling to pick up her children from school.
Unarmed Pc Keith Palmer died after being stabbed at the Houses of Parliament.
Five people remain in critical condition and two have life threatening injuries.
Police said nobody living at the house was a suspect and are appealing for information as they try to establish whether Westminster attacker Masood acted alone. We will never be cowed by terrorism.
However, he was known to police and MI5 and had convictions for assaults, including GBH, possession of offensive weapons and public order offences.
"Masood was not the subject of any current investigations and there was no prior intelligence about his intent to mount a terrorist attack", the police said in a statement.
Meanwhile, Islamic State made its first public pronouncement since the atrocity, claiming in a statement: "The attacker yesterday in front of the British Parliament in London was a soldier of the Islamic State executing the operation in response to calls to target citizens of coalition nations". The Birmingham Central Mosque reported an increase in hate messages after the attack by Masood, who reportedly had links with Birmingham.
Last night Trafalgar Square filled once more - this time there was no dancing, no music and no celebrating.
The Mayor joined by Home Secretary Amber Rudd and Metropolitan Police Deputy Commissioner Craig MacKey, invited both tourists and Londoners to pay their respects to the victims and the come together in solidarity against terror.
Al Jazeera's Barnaby Phillips, reporting from London, said: "The absolute priority of the police at this point in time would be to know what sort of accomplices, if any, the assailant had".