The group of fighters and coaching staff, which included Rio Olympians Muhammad Ali and Pat McCormack and experienced super-heavyweight Frazer Clarke, were in Westminster attending a community event at the Houses of Parliament.
The Lionhearts were in the capital ahead of their World Series of Boxing match against Italia Thunder at York Hall on Thursday night, a fixture which will go ahead as scheduled.
A statement read: "GB Boxing can confirm that boxers and coaching staff from its British Lionhearts boxing team were at Westminster and witnessed part of the attack at the Houses of Parliament.
"No members of the British Lionhearts team or coaching staff were harmed in the incident and all of the boxers and staff are safe and well.
"The boxers and staff have spoken to the police and security forces and are assisting them with their enquiries into the incident.
"The boxers are due to compete in the World Series of Boxing against Italia Thunder and are very keen to fulfil this despite (Wednesday's) events.
"The team is completely focused on preparing for the fixture and will not be making any further comment until after the match is over.
"The boxers, coaches and everyone connected with GB Boxing and the British Lionhearts would like to express their sympathies to everyone that was hurt in (Wednesday's) incident and their sincere condolences to all of the families that have lost loved ones."
The Lionhearts were being hosted by the All Party Parliamentary Group for boxing at an event to highlight the achievements of the British Lionhearts in the Community programme.
Clarke posted a message about the events on his Instagram account.
It read: "Yesterday a sad, sad day for everyone involved. My thoughts to everyone involved and their loved ones.
"A long day for me and the team but we are still here and will return to our loved ones. Others won't. I appreciate the messages of concern but I'm fine and focused on tonight and getting my job done.
"Just a quick one to everyone; let's not live in fear over the act of a crazy person. Let's not judge others by this man's actions. This isn't down to religion, because this crazy act is by someone that has no faith at all.
"We are well looked after. I (have) seen it first hand, still the greatest city in the world and we should be proud of it."
