AMSTERDAM — To the cheers of tens of thousands of people crammed shoulder to shoulder outside the royal palace here, Willem-Alexander of the House of Orange-Nassau became the Netherlands’ first king in 123 years on Tuesday as his mother, Queen Beatrix, ended a 33-year reign with the stroke of a pen, signing the act of abdication in an ornate chamber at the palace.

Seated on a gilded chair and flanked by the Dutch cabinet, Beatrix, 75, became the third successive Dutch queen to abdicate, changing her title to princess as supporters celebrating the continuity of the monarchy thronged Dam Square outside the palace. Many had gathered for hours, clad in orange, the royal color, to watch the brief, long-planned and relatively low-key event on large television screens.

The day’s events, a mix of tightly choreographed official pageantry and boisterous street parties across Amsterdam, helped lift the gloom created by a slumping economy, unpopular austerity measures and growing public recognition that, as the new king noted in an afternoon address, “it now seems less self-evident that the next generation will be better off than the last.”