MANAMA, Bahrain—Hundreds of hard-line Bahraini opposition protesters gathered in Manama's financial center on Monday, defying a police order to disperse, in a new escalation of tensions between antigovernment demonstrators and the ruling Al-Khalifa family.

A small number of protesters pitched tents Sunday night at the base of two skyscrapers in the heart of the financial center, but were warned by police to move on, demonstrators said. Amid fears of a crackdown, the number of protesters began to swell, with hundreds calling...