The claims come as an RAF transport plane carrying Britons seriously injured in the shooting landed at Brize Norton, after a brief stop at Birmingham Airport.
David Cameron has declared a national minute’s silence for victims of the attack, while Tunisian interior minister Mohamed Gharsalli announced the arrests of several nationals allegedly linked to attacker Seifeddine Rezgui.
Authorities say Rezqui acted alone during the rampage in the resort of Sousse but had accomplices who supported him before, providing him with weapons and logistical support.
But British tourists staying in the north African country claim they saw a second man carrying a gun and roaming the resort.
Steve Johnson, a retired police officer, was on holiday with his wife Valerie and fellow ex-officer Michael Perry and wife Angela.
He told the Daily Mirror: “We were on the beach when I heard gunshots and screaming and I told people to ‘run’ and got off the beach as fast as I could. We were really close and I could hear the constant noise of an automatic weapon. I saw a man in red shorts machine-gunning someone on a sun lounger. He was 20 yards away from us.
“We ran across wasteland and into a spa and hid in the basement while gunshots and grenades and everything was going off around us. After about 20 minutes we crept up to the third floor and we could see him shooting people around the pool. It was 40 minutes later when a Tunisian policeman came to get us and he took us through a corridor from the spa to the hotel.
“On the floor was a body, covered mainly with a sheet, but I could see he was the right build and colouring of the shooter I had seen. And he had red shorts on ... Mick had seen both gunmen and we are sure there were two of them.”
Mr Perry told the paper he had seen the bare-chested gunman running with a weapon.
When he asked a police officer what had happened to the attacker, he was reportedly told: “We shot him”.
Rezqui, dressed in black, appeared to be alone in the video footage of the atrocity that has emerged so far.
The seven people in custody were arrested in at least three different cities and taken for questioning in the capital Tunis.
Yesterday, Mr Cameron announced there would be a nationwide minute’s silence at midday on Friday, while Home Secretary Theresa May condemned the “despicable act of cruelty” after she visited the scene of the attacks.
A total of 38 people, including up to 30 Britons, died after 23-year-old student Rezgui opened fire on Friday. The self-proclaimed Islamic State (IS) - also known as Isil - later claimed responsibility.
Announcing the minute’s silence, Mr Cameron told MPs: “I know the whole country will want to share in a moment of remembrance.”
The Prime Minister, who earlier described the “existential threat” posed by IS as “the struggle of our generation”, told the Commons: “We will not give up our way of life or cower in the face of terrorism.”
He added: “This is not the war between Islam and the West which Isil want people to believe - it is a generational struggle between a minority of extremists who want hatred to flourish and the rest of us who want freedom to prosper - together we will prevail.”
He said Britons were not being advised to stay away from Tunisia’s coastal resorts despite the bloody events at Sousse, while he also disclosed that a major exercise will take place in London this week to ensure the UK is prepared to deal with terrorism.
Mrs May laid flowers and observed a period of silence at the scene of the attack and held talks with Tunisian, German, French and Belgian ministers on addressing the threat posed by IS.
Downing Street said all British nationals injured would be returned within the next 24 hours.
Four people needing treatment have been flown back to the UK on board an RAF C17 plane with “medevac” teams experienced at bringing injured service personnel back from operations overseas.
The Ministry of Defence confirmed the plane had landed at RAF Brize Norton in Oxfordshire, after dropping one patient off at Birmingham Airport.
“Patients have now been transferred over to the care of NHS England,” a spokesman said.
The British Red Cross has sent four specialist psycho-social volunteers to support British holidaymakers caught up in the attack and has opened a support line for people in the UK on 0844 4631 345.