KASHGAR, China — Police tightened security in China's western Xinjiang region today and Olympic organizers sought to reassure residents and visitors after a deadly attack on police heightened jitters just days ahead of the games' opening ceremony.

China's official Xinhua News Agency reported that authorities reinforced police checkpoints at roads leading into Kashgar, the scene of yesterday's attack, with police boarding buses to search passengers' bags. A full security alert has been issued in public places, including government office buildings, schools and hospitals, the agency said.

In Xinjiang's capital of Urumqi, police used handheld devices to check residents' security cards in routine street patrols.

Sixteen officers were killed and another 16 were injured when two assailants rammed a dump truck and hurled explosives at a group of jogging policemen in Kashgar.

The attack in the city near the Afghanistan-Pakistan border brought an immediate response from China's Olympic organizers, who said security precautions are in place to ensure safety in Beijing and other Olympic venues when the games open Friday.

The timing so close to opening day heightened the attack's shock value.

"Security for the Olympic Games is of paramount importance. The more we give, the safer and more secure the residents will feel," Xinhua quoted Han Shubin, the deputy director of an Urumqi police division, as saying.

Today, the streets in northwestern Kashgar were quiet, though four soldiers in uniform and helmets marched up the sidewalk on patrol, carrying short, black clubs.

"I heard the attack yesterday morning. It was not loud. It just sounded like a car's tire bursting," said a waitress who worked in a small nearby restaurant who refused to give her name for fear of reprisals.

"It doesn't seem like there are more police on the streets, but the neighborhood patrols seem to be tighter. They're coming around and checking more often," she said.

Underscoring tensions in the region, Chinese authorities clashed late yesterday with two Japanese journalists who rushed to Kashgar to report on the attack, Xinhua and Japanese officials said. Authorities later apologized after Tokyo said it would lodge a formal complaint.

The clash occurred when the journalists tried to film a restricted area, Xinhua said. Japan's Chief Cabinet Secretary Nobutaka Machimura told reporters in Tokyo that the two had been detained and roughed up.

"We plan to lodge a strong protest," Machimura said.

Authorities have said they suspect terrorism was behind the attack. Last month, an extremist Uighur group believed to be based across the mountainous border in Pakistan's tribal frontier threatened to target the Olympics.

The two suspects arrested were Uighurs, a mainly Muslim ethnic minority group that has waged a sporadically violent rebellion against Chinese rule.

State broadcaster China Central Television said in its noon broadcast today that the two men, aged 28 and 33, had planned the attack, stealing the dump truck and ramming it into some 70 border patrol paramilitary police as they passed a hotel during a morning jog. They then hurled the explosives and attacked the policemen with knives, CCTV said.

One of the attackers lost his hand when the homemade explosives detonated. Afterward, police recovered additional explosives, a gun and "contents about a holy war," CCTV said.

China has made safety a major priority for the Summer Games, mobilizing hundreds of thousands of police, military and local residents as part of a huge security net over the capital. But the Xinjiang attack underscored that, with so much security focused on Beijing, areas far from the capital make tempting targets.

The assault took place on a treelined thoroughfare in front of the small Yiquan Hotel, housed in an older three-story building covered in dusty white, yellow and maroon tile.

The hotel was closed today, with a large plastic tarp covering the entrance. Just to the right of the hotel, a group of four trees appeared to have been recently uprooted.

Monday's attack was all the more surprising because it follows years of intensive security measures in Xinjiang. A wave of violence in the 1990s mainly targeted police, officials and Uighurs seen as collaborators. Separatists also staged nearly simultaneous explosions on three public buses in the provincial capital of Urumqi.

In response, the government stationed more paramilitary units in the region and shut unregistered mosques and religious schools seen as hotbeds of anti-government extremism.