China has tightened visa controls ahead of the Olympic Games, with invitation letters and hotel reservations required for some travelers, the government acknowledged Tuesday after weeks of insisting the entry process was business as usual.

"We have made some arrangements according to the practice of the past Olympics and usual international practice. That is, in the approval process we are more strict and more serious with the procedure," Foreign Ministry spokesman Qin Gang said.

The changes come amid fears the summer games will be disrupted by protesting activists or possibly terrorists -- a concern deepened after the troubled progress made by the international Olympic torch relay.

The flame's round-the-world tour was dogged by protests in London, Paris and other cities where demonstrators voiced their disapproval at Beijing's clampdown on a broad uprising among Tibetans against Chinese rule.

In a country where control has long been paramount, China is facing unprecedented challenges when an estimated half million visitors come to Beijing in August for the Olympics, along with 10,500 international athletes and 18,000 journalists.

There have been numerous reports of foreigners being kicked out of Beijing because of tightening visa restrictions. The government seems to be targeting students, apparently fearing some might mount pro-Tibet demonstrations during the games.

But travelers, bloggers, foreign embassy officials and travel agents are also complaining that they are having trouble getting visas to stay in the country during the Olympic period.

Qin said the policies would "be maintained for a period of time," but did not give further details.

However, the official Xinhua News Agency said Tuesday that extra checks on travelers could include providing invitation letters, proof of accommodation and round trip air tickets. It cited a statement from the consular department of the Foreign Ministry.

It also said visa applicants are encouraged to apply in their country of citizenship. Many foreigners used to apply for visas in Hong Kong.

China has said it faces threats from Tibetan separatists overseas seeking to disrupt the games, branding one Tibetan group a terrorist organization. An attempt in March to hijack an airplane was blamed on Islamic separatists among the Uighur population in the western region of Xinjiang.

Activist groups critical of China's human rights abuses and its rule over Tibet have pledged to try to disrupt the games.

Travel agents in Hong Kong, a major gateway into China, reported last month that the government visa office had declared multiple-entry business visas would not be available from mid-April until mid-October. In the past, such visas were easily obtainable.

Adding to the confusion was China's continued insistence that no changes had been made to its visa policy.

Qin said it was not true all multiple-entry visas had been suspended. He did not give further details.

"What is unchanged and will not change is safeguarding our national security and making sure our environment is safe and making sure foreigners in China will be safe," he said at a news conference.

Both the American and European chambers of commerce in Hong Kong sent letters to the Chinese government last month, citing several cases of visa denials and raising concerns over the impact on business.