Tens of thousands of people massed in Moscow yesterday, waving flags and banners demanding ``Power to the People, Not the Party.'' The Soviet authorities have warned the pro-democracy rallies could lead to mass disorders. Police were out in force but made no attempt to interfere with the demonstrations. According to police estimates, 35,000 people had gathered in Moscow. The crowd was considerably smaller than expected. Hundreds of thousands had been expected to fill Moscow's streets in a protest similar to a demonstration by 200,000 next to the Kremlin on Feb. 4.