Since Mr Masari arrived in London - an exile - last year, the modest suburban premises of his movement have become the focus of opposition to the royal family of Saudi Arabia. The Saudi government alternately denounces Mr Masari as a menace or dismisses him as irrelevant. Its ambassador in Britain, Ghazi Algosaibi, has become inured to pamphlets and vigils outside his Belgravia embassy calling for holy insurrection.
But Mr Masari's activities have furrowed brows in the Foreign Office, whose representatives are on the receiving end of Saudi royal displeasure. Britain's ambassador in Riyadh, David Gore-Booth, is believed to have argued for a tough line against Mr Masari and in favour of unflinching British support for the House of Saud.
"Ah, Mr Gore-Booth," mused Mr Masari. "Well, he is doing his job properly - in your short-term interests, that is." Mr Masari takes the longer view. "I have regarded myself as a revolutionary since the 1960s. I am very patient. But I do believe this is the job of one generation. Look, the Iranian movement took only 30 years - it can be done in one generation: the seeds sown and the harvest brought in.''
Mr Masari is 48, with two wives and eight children. He is best known as the man behind the Committee for the Defence of Legitimate Rights (CDLR), which promotes itself as a pressure group for the improvement of human rights and peaceful reform in Saudi Arabia. But the CDLR, Mr Masari said, is a broad church which encompasses people of various persuasions united by dislike of the Saudi royals. He himself, as a revolutionary militant, does not disguise its utility.
"I believe in pulling the regime out by the roots," Mr Masari said. In the course of two hours' conversation he comes across as a man whose Muslim religious integrity is absolute but whose political method is Leninist. He speaks of his network of informants in Saudi Arabia, men who adopt codenames and professions on the phone to north London, people "almost up to the fringe of the royal family, up to brigadier rank in the army and very high-ranking in the secret police". He quotes Mao on paper tigers. And his next task is the drafting of what he acknowledges could be seen as an Islamic Manifesto.
Mr Masari was professor of physics at King Saud University in Riyadh when he joined a group of prominent religious and academic thinkers to form the CDLR in 1993.
At first their petitionings and complaints were accorded the traditional royal tolerance . But when they continued to make allegations of corruption, the secret police were called in. Mr Masari was arrested and was, he says, tortured. So were dozens of others. Eventually he escaped into exile. The CDLR hands out a list of 277 people it says are political prisoners, many of them clergymen and scholars. "The Al-Saud," said Mr Masari, "are at war with the cream of the country.''
It remains impossible to verify the committee's claims of widespread subversion or, indeed, to contradict the government's assertion that everything is tranquil. An unusual number of executions by the sword this year may testify to little more than a recessionary crimewave and a determination by the authorities to prove that their zeal for Islamic punishment is beyond challenge.
But, just in case Mr Masari turns out to be right, his account of how the Saudi opposition grew up is worth careful study.
"Of course, in the boom years after 1973 the people had so much that they didn't mind the corruption," he said. But a group of Islamist thinkers had come to conclude that the rule of the House of Saud was illegitimate. "Any regime based on inheritance and lineage cannot be Islamic at all," he said. The contract in Islam between ruler and subject, known as bayah, had been breached.
Then in 1979 came the Islamic revolution in Iran. Its impact on Saudis, said Mr Masari, was enormous. At the same time the long war in Afghanistan - "a clear case of jihad" - had great influence. "A large number of young people went to fight, learned to use weapons and found their own independence," Mr Masari recalled. Saudi society, smug and protected by the West, offered little stimulus to such returning adventurers.
Meanwhile, the oil price declined in the 1980s while the value of the dollar, in which revenues were denominated, also fell. Saudi Arabia went from cornucopia to austerity. In 1990 foreign troops flooded into the kingdom. "The Gulf war was the big explosion," Mr Masari said. Despite the expenditure of billions of dollars, the House of Saud could not defend the realm. Disillusion became fashionable; dissent flowered.
Mr Masari does not predict riots in the streets but "palace wars, perhaps a coup or an economic crash". He claims to prefer a slow process to give the Islamists time to build. "If they collapse quickly then the military role will be greater than we want to see," he said.
His Saudi Arabia would be purely Islamic, non-aligned, free of foreign forces although, he says, hospitable to foreigners; stern in its maintenance of sharia law punishments, strict to enforce female modesty through the veil but firmly upholding women's right to economic equality. Mr Masari asserts that Shia Muslims and other minorities would be respected.
Some suspect that Mr Masari may be a pawn in a royal intrigue, perhaps between Crown Prince Abdullah and the "Sudairi Seven" - King Fahd and his six full brothers. Funds for his movement, though mysterious, are evidently not a problem. And for all the indignation of Douglas Hurd, the Foreign Secretary, and John Major, Mr Masari says "we have had barely any pressure from the British Government". Perhaps they are quietly hedging their bets.