Cumbrian MP Tim Farron said he could not do his job from behind a locked door.
Calls for extra security and a new way of working for MPs have been made following the fatal stabbing of Conservative MP Sir David Amess at a surgery with constituents on Friday.
A 25-year-old man is currently being held under the Terrorism Act.
Mr Farron, a previous leader of the Liberal Democrats, said in the immediate aftermath, he was thinking of Sir David and his family.
He said the situation also impacted office staff who worked for MPs and their families.
He said: “As a Christian, I prayed for Sir David and his family. As MPs, we cannot be cowed by this. We cannot be hidden behind locked doors.
“This is an assault on democracy and if we have to dial down democracy, then I dont want to be an MP.
“On Friday afternoon, I carried on with an outdoor street surgery and on Saturday, I held my normal surgery.”
At Saturday’s surgery, there was a police presence at Mr Farron’s office.
He added: “The police have been excellent and it was on their advice that we have a police presence.
“They asked if I wanted someone in the room with me but I didn’t want that. While I trust the police, my constituents are coming to see me about issues in confidence and they don’t want someone there ‘listening in’ – even though the police would not take it any further.
“The police presence isnt something I’ve demanded – we’ve listened to their advice about the best course of action.”
Mr Farron said he had received serious and credible death threats in the past.
“Yes, there have been death threats and other unpleasant things but its outweighed by the absolute privilege of serving people.
“As an MP I’m immersed in the community and I can’t do the job any other way.”
