LONDON — The latest and perhaps most dangerous challenge to the House of Saud is a portable telephone that Mohammed al Massari carries in his pocket around the fashionable hotel lobbies, restaurants and offices of London.

It rings every 10 minutes or so, and it's always a caller from somewhere in Saudi Arabia--dialing through a New York exchange so the call can't be traced--with tidbits of damaging gossip about the royal family, a report on the arrest of an opponent of the Saudi regime or simply a word of encouragement. "Thank you for what you're doing," said one caller from Riyadh on a recent afternoon. "We all support you."

Al Massari works for the Committee for the Defense of Legitimate Rights, whose office in London is a virtual guerrilla warfare center, a network of computers, files and fax machines that transmit reams of sedition into the remote desert kingdom, one of the most closed societies on Earth.

The London operatives say 300 faxes a month go to Saudi government offices (the military was recently ordered to keep fax machines turned off unless a specific document was expected), and 300 more go to various merchants, intellectuals, businessmen and clerics who in turn copy them and re-fax them, in numbers totaling more than 100,000, throughout the kingdom; the committee claims, with great delight, to have recently acquired the number to the fax machine in the bedroom of Saudi Defense Minister Prince Sultan ibn Abdulaziz, brother of King Fahd.

In recent weeks, Prince Sultan might have awakened to find these bits of propaganda at his bedside: that Saudi Arabia was secretly supplying the southern Yemenis with tanks and other military assistance in neighboring Yemen's civil war; that the number of pilgrims who died in an accident during the recent hajj pilgrimage was 10 times more than the government reported, or more than 2,200; that King Fahd and his brothers had in April received salary payments of 100 million Saudi riyals, about $27 million.

To the worry of both the Saudi monarchy and many Western governments, this particular opposition organization is an Islamic group bent on making the rigorously Islamic nation even more fundamentalist. Yet its calls for democratization and an end to corruption have attracted the interest of Saudi liberals pushing for greater freedom and public participation in government.