Commuter Anna Dunwoodie told the inquest into the Brazilian's death how she was "very, very clear" officers did not shout armed police before opening fire.
Ms Dunwoodie also said she thought firearms officers were a gang, as she described a "sense of panic" from officers as Mr de Menezes was shot seven times in the head.
Revealing how she felt under pressure during initial interviews with police after the incident, she described how the innocent 27-year-old appeared calm as a gun was held to his head.
Ms Dunwoodie was sat two or three seats to the left of Mr de Menezes when he boarded the train at Stockwell Tube station, south London.
She said she never heard officers shout anything at Mr de Menezes, adding: "I would like to say that on whether I heard anything from police officers, I am very, very clear.
"I had absolutely no idea who they were and had they shouted I would have latched on to that."
She described scenes of panic on the carriage.
Ms Dunwoodie told a jury at the Oval cricket ground: "I think it was the man, who I now know to be a surveillance officer, (who) really seemed to be frightened or hyped up and when he was calling the other men they seemed... you know, when people are full of adrenalin and they move quickly and their movements are a bit jerky.
"I felt they were a bit out of control, that's what it felt like."
Mr de Menezes was shot at point-blank range at Stockwell Tube station on 22 July 2005 after being mistaken for failed bomber Hussain Osman.
Mr de Menezes had his eyes closed and looked "almost calm" as firearms officers pointed a weapon at his head, Ms Dunwoodie said.
She added: "It was sort of a scrum. I remember the man holding the gun out.
"His (Mr de Menezes's) eyes were closed and he looked almost calm, although I hesitate to say that.
"I guess he had a gun pressed to his head and there was not anything he could do about it.
"The first shots came very suddenly and my first thoughts were that it was someone firing a stun gun.
"There was a break and it was my memory that there were more shots."
Before the shooting, police were "shouting amongst themselves", she said.
Referring to failed attacks on London the previous day, Ms Dunwoodie added: "It was a particularly strange day in London. I did not know who they were. I thought it was a gang.
"There was a sense of lots of men, lots of guns, everything was so fast."
Ms Dunwoodie went on to explain how she felt under pressure during interviews with police while she was still "shaken up" afterwards.
Recalling giving a statement to police, she said: "I was really under pressure to look at a memory that was very recent and frightening."
A surveillance officer, using the code name Ivor, previously told the jury how Mr de Menezes stood up and walked towards him as police challenged the Brazilian.
But Ms Dunwoodie, who was reading a book on the train before the incident, said she had no recollection of Mr de Menezes doing anything.
She said she was "most frightened" of Ivor, who was also sat near her, before the shooting.
Ms Dunwoodie, who had boarded the Northern Line carriage at Tooting Bec station, added: "He did make me feel very nervous.
"My attention was drawn to him. He seemed to have stuff in his bag and there seemed to be a metallic noise.
"He then ran to the door opposite."
She later heard Ivor shouting "there he is" as he pointed to Mr de Menezes.
Ms Dunwoodie added: "I thought he (Ivor) was my main threat."
After the shooting she told how she thought "something illegal" had happened.
Ms Dunwoodie said: "Our first impulse was to call the police because we needed to tell the police something illegal had happened."
Inquest records showed she was sitting three seats away on the same row as Mr de Menezes - but she said later that there was a gap of two.
Two eyewitnesses who sat near the carriage doors where firearms officers opened fire also said they had no recollection of any warnings from armed police.
Robert Preston said he only heard police shouting "get down, get down."
"It was quite a general statement and it could have applied to me," he said.
Mark Whitby, a fellow passenger, added: "Get down, get down. That's all I heard.
"There was not one mention of armed police."
