One of the alleged July 21 bombers said the plan to detonate rucksack explosives on London's transport network was a "stupid idea" which he regretted.
Ramzi Mohammed, 25, the so-called Oval bomber, was caught on CCTV sprinting from the train.
He has admitted setting off the device but insists it was only supposed to make a popping noise and was in protest against the war in Iraq.
But when it caused chaos and led to his arrest, he realised this had been foolish, Woolwich Crown Court was told. He said: "It was a stupid idea, I regretted it. It did not help (attitudes towards Muslims), it made it worse."
The jury was told that Mohammed believed that in Islam killing was wrong and he rejected the concept of suicide bombing.
He denied writing Arabic script on his cell wall because he said he was not very good at Arabic. He told the court he had taken a part-time course at Southwark College but left because it was too difficult. He said he did not know who had written the Arabic writing in his cell. "I could not read it because it was too hard, it was too advanced for me."
One of the inscriptions said: "Al Qaeda is a book that guides and a sword which gives victory."
Mohammed said the words were un-Islamic and when asked what he thought about Al-Qaeda, he replied: "They are a group of terrorists who live in the mountains."
Mohammed is among six men accused of taking part in a plot to carry out a series of suicide bombings.
The others are Yassin Omar, from New Southgate, north London; Manfo Kwaku Asiedu, 34, of no fixed address; Muktar Said Ibrahim, 29, of Stoke Newington, north London; Hussain Osman, 28, of no fixed address; and Adel Yahya, 24, of High Road, Tottenham, north London.
