JOHANNESBURG — The police in Swaziland, the last absolute monarchy in Africa, squelched a long-planned pro-democracy rally on Tuesday, firing water cannons and tear gas into crowds in Manzini, the nation’s largest city.

Organizers had dubbed the protest “the April 12 Uprising,” recalling the day, 38 years ago, when King Sobhuza II had abandoned the country’s British-style Constitution and rid himself of the inconvenience of political parties.

His son, Mswati III, is now king. Some things have changed, some not. The old king had more than 70 wives, the new one 14. Mswati III is one of the world’s richest monarchs, and he provides most of his spouses with their own retinue, a palace and a new BMW; two-thirds of his 1.2 million subjects live on less than $1 a day. They have the world’s highest H.I.V. infection rate.

There is again a Constitution, instituted in 2006, though not one assuring political freedom. A pro-democracy movement has existed for decades, but it has had more fits than starts. Many of its leaders are routinely jailed, and on Tuesday, this was done peremptorily with the morning arrests of the trade unionists at the vanguard of the rally.