Sir David Amess, the MP for Southend West, died after he was stabbed 21 times during a constituency surgery in Leigh-on-Sea, Essex, on 15 October.
Ali Harbi Ali, 26, on trial at the Old Bailey, denies murder and preparing acts of terrorism.
Julie Cushion, a long-term member of Sir David’s team, told the jury she heard a “piercing scream”.
The court heard Ms Cushion had been the first to call 999 from the scene at 12:07pm
Sir David was attacked in a back room of Belfairs Methodist Church during an appointment made by the defendant, the prosecution said.
Giving evidence from behind a screen, Ms Cushion said: “A piercing scream was the first thing I heard. Then a very loud crash which I assumed was furniture going over.
“I got up and walked through, I assumed something structurally had happened.”
She said that another aide, Rebecca Hayton, ran from the room “screaming that Sir David had been stabbed”.
“At that moment I picked up my phone to call for help,” Ms Cushion added.
Asked how Mr Ali looked, she told the jury: “Self-satisfaction, I would describe it.”
The jury also watched a police interview with Ms Hayton hours after the attack where she described seeing Mr Ali stand up and stab Sir David.
She said: “He [Mr Ali] leans over and stabs him straight in the stomach.”
“I saw it [the knife] go in once and as I got up he [Mr Ali] kept going,” she added.
Ms Hayton said Mr Ali had been “smiling” during the five-minute meeting with Sir David, adding: “He seemed relaxed and nice.”
Following the attack, she ran outside the church to raise the alarm and returned with Yvonne Eaves and Darren King.
Sir David was lying prone on the floor and Mr Ali was standing in the doorway clutching a knife, she told police.
She said Mr King tried to talk to Mr Ali but the defendant said “don’t come any closer”.
Ms Eaves told the court that when she arrived at the scene two women were outside looking “really frantic.
She said at one point Mr Ali was pacing and “seemed like he knew what he’d done. He wasn’t remorseful,” she said.
The trial previously heard how Mr Ali had spent years plotting an attack, researching a range of potential victims including high-profile politicians Michael Gove, Dominic Raab and Sir Keir Starmer, before choosing the 69-year-old backbencher.
Ms Cushion said Sir David had moved the meeting from his constituency office,
where there was security, into the community after coronavirus restrictions were lifted because he “wanted to be accessible to his constituents.
Mr Ali had claimed to be a healthcare worker moving to the area who “wanted to get to know his new MP”, aides were told.
On Tuesday, jurors were played a 999 call from the aftermath of the attack where a witness said Mr Ali was threatening to kill four other people.
They were also shown CCTV footage of Mr Ali making the journey from his home in Kentish Town, north London, to the appointment in Leigh-on-Sea, by train and on foot along busy high streets and residential areas.
The court heard Mr Ali had a “warped” ideology and sought to exact “revenge” over Britain’s recent foreign policy, notably bombings in Syria in the mid 2010s, allegedly telling bystanders at the scene: “I want him dead. I want every Parliament minister (sic) who signed up for the bombing of Syria who agreed to the Iraqi war to die.”
