THE PORTLAND CENTER FOR THE PERFORMING ARTS in Oregon opens with a gala two-week celebration, starting tonight, which signals the end of a 10-year community project to provide first-class performance space for the arts. Two new theaters now join the Arlene Schnitzer Concert Hall on the complex site. Tonight's ``Broadway Event'' offers outdoor singing and dancing, Philippe Petit's high-wire act, and a laser show finale. Free events include ``Spectrum of Black Music'' (tomorrow), ``The Great Northwest Blues Jam'' (Sunday), ``Brown Bag'' reggae, jazz, folk, and comedy (weekdays), and children's theater, dance, and juggling (Sept. 7). Among ticketed events are ``Sunday in the Park with George,'' Smokey Robinson, the Flying Karamazov Brothers, the West Coast Chamber Orchestra, and Ballet Oregon. THE HOLLAND FESTIVAL OF EARLY MUSIC in Utrecht starts today, with some 60 main concerts and over 100 other activities (lectures, video performances, and an Early Music Fair) in historical venues and the Vredenburg Concert Hall. Concerts include performances by the Amsterdam Baroque Orchestra, the Consort of Musicke, Les Arts Florissants, the New London Consort, Gustav Leonhardt, Ton Koopman, Esther Lamandier, and Nigel Rogers. Festival ends Sept. 6. THE 10TH ANNIVERSARY Alternate Roots Festival in Atlanta, Sept. 16-20, will offer original works by Southeastern companies and independent artists. Also featured are the Roadside Theater; the Small Family Orchestra; Atlanta's Seven Stages Theatre; choreographers Celeste Miller, Lesley Neal, and Louise Udaykee; and new vaudevillians Kenny Raskin and Bill Snyder.