China's top leaders opened a special Communist Party conference on an optimistic note today, calling for a continuation over the next five years of market-oriented economic reforms and pledging to name scores of new, younger leaders to replace aging revolutionaries in the party's top ranks.

Presiding over the conference with several other senior leaders, Party General Secretary Hu Yaobang told the 992 delegates that the process of promoting younger leaders within the party has been "quite smooth," according to the official New China News Agency.

But an unusual demonstration today at Peking University served as a reminder of tensions that still exist within Chinese society. On the 54th anniversary of the Japanese invasion of northeast China, more than 700 students demonstrated against what they called signs of a revival of militarism in Japan.

In his speech to the delegates, General Secretary Hu said the average age of members of the Communist Party Central Committee will be "significantly lower" as a result of resignations by 64 older cadres from the Central Committee.

The resignations were announced Monday after a secret five-day plenary session of the Central Committee.

Hu disclosed that 56 new members and 34 alternate members are to be elevated to the Central Committee at the current conference. This means an expansion of the Central Committee, which currently comprises 210 full members and 136 alternates.

At a briefing for reporters, a spokesman for the conference, Zhu Muzhi, said those elderly officials who are retiring will retain their full salaries and special privileges, such as the use of automobiles and spacious housing.

On the economic side, Premier Zhao Ziyang told the delegates that the major tasks being proposed for the next five-year plan, which begins in 1986, included upgrading the country's technology and further raising living standards.

In China's cities, Zhao said, efforts should be made to make large and medium-sized state-owned enterprises "independent and responsible for their own profit or loss."

He advocated "gradually reducing state control over enterprises and establishing indirect control."

Some attention was diverted from the conference opening when several hundred students reportedly left the campus of Peking University, on the outskirts of the city, and reportedly staged demonstrations in Tiananmen Square, near where delegates to the party conference have been meeting.

The protest was ostensibly called to commemorate the Mukden incident, which occurred on Sept. 18, 1931, when Japan invaded northeastern China. It was the memory of this invasion and the subsequent Japanese occupation, as well as current resentment against Japan for flooding the Chinese market with Japanese consumer products and creating a severe trade deficit, that galvanized the students.

Some of the students said they were critical not only of the Japanese government but also of the Chinese government, which they accused of being too accommodating toward Japan.

What seemed to offend the students the most was a visit last month by Japanese Prime Minister Yasuhiro Nakasone to a shrine in Tokyo that honors Japan's war dead. Several students said they thought the Chinese government had not been forceful enough in protesting the visit.