NAIROBI, KENYA, JULY 7 -- Paramilitary police fired on a banned rally for multi-party democracy here today after stone-throwing demonstrators attacked a security agent and commandeered a city bus.

The violence, which closely followed the arrests of prominent proponents of a multi-party system, underscored the political tensions that have risen quickly in this one-party state, long considered one of Africa's most stable and successful nations. Since the beginning of the year, the government of President Daniel arap Moi has come under increasing pressure to permit political pluralism.

Journalists and other witnesses did not report seeing anyone hit by gunfire. However, dozens of people were injured when they were struck by truncheon-wielding police. There were no reports of deaths, and no immediate word on the number of arrests.

By nightfall an uneasy calm settled over the city, with riot police stationed at many street corners, occasionally striking lone pedestrians.

Proponents of multi-party democracy had requested permission to meet today at the Kamakunji meeting grounds, set amid slums a few blocks from Nairobi's downtown. Moi's government vowed to prevent the gathering, with officials threatening to arrest anyone who appeared.

The authorities appeared successful in their bid to quash the rally until midday, when several hundred people gathered on the meeting grounds. Thousands more stood watching as those on the grounds waved two fingers in the air -- a symbol of support for multiple parties -- and chanted "Free Matiba." Kenneth Matiba, a former cabinet minister who has led the campaign for a multi-party system, was arrested on Wednesday.

Security forces stood by until the crowd stoned a plainclothes security agent and his car and commandeered a city bus. According to witnesses, troops fired guns and tear gas. Three truckloads of crack riot police quickly arrived.

As the crowd fled, scattered looting occurred in areas near Kamakunji. Hours later, glass, stones and other debris littered the streets, and rock-bearing crowds stood facing lines of riot police, a few yards of open road separating them.

In a commercial district several hundred yards away, pedestrians scattered in nervous flurries before advancing police. Truckloads of riot police moved through the streets, firing from open trucks into the air and yelling at bystanders to disperse.

The disturbances came at the end of a week in which police detained at least 40 people in a crackdown on those believed to favor a multi-party system. Of those arrested, 29 were accused of possessing subversive music cassettes that bore anti-government lyrics.

Prominent among the other 11 were lawyers critical of the government; Matiba, the former cabinet minister; Charles Rubia, another former cabinet minister who had joined Matiba in public calls for change, and Raila Odinga, who previously was twice detained and is the son of Kenya's first vice president.

The government's suppression of dissent drew criticism from the U.S. State Department, which said in a statement Friday that the detentions could "only serve to tarnish Kenya's image." It urged the government to permit freedom of expression and assembly.