LONDON, Oct. 26 — A modest two-story red brick house indistinct from others along a North London street bears no signs of being the headquarters of a campaign challenging royal rule in Saudi Arabia. The marks of the exiles' activity are very noticeable, however, in the streets of Saudi Arabia itself, where hundreds of people have been arrested by antiriot police this month for taking part in a protest demonstration, a rare occurrence in the monarchy's history. Government alarm at the dissidents' influence was reflected Sunday in a Ramadan-eve address in Riyadh from the country's senior religious leader, the grand...