Japanese Prime Minister Junichiro Koizumi has reiterated his country's "deep remorse" over its colonial aggression in Asia.
Koizumi hopes to meet China's President Hu at the summit
The speech at the Asia-Africa summit comes amid tensions over the approval by Tokyo of school textbooks which China says gloss over Japan's record.
Mr Koizumi later said he hoped to meet Chinese President Hu Jintao on Saturday - although Beijing has yet to agree.
Leaders from 80 nations are attending the two-day summit in Indonesia.
The annual Asia-Africa summit has increasingly been dominated by global trade issues.
But the escalating row between China and Japan threatens to overshadow all other considerations.
Addressing delegates, Mr Koizumi said: "In the past Japan through its colonial rule and aggression caused tremendous damage and suffering for the people of many countries, particularly those of Asian nations.
"Japan squarely faces these facts of history in a spirit of humility."
The wording repeats previous Japanese apologies - but analysts say the international setting gives the statement added weight.
Asked by reporters if he would hold talks with President Hu, Mr Koizumi said he was hoping for a meeting on Saturday.
The BBC's Tim Johnston in Jakarta says the apology should go some way to placating Chinese anger, which was recently reignited by a history textbook that the Chinese felt paid insufficient attention to atrocities.
Chinese Foreign Ministry spokesman Kong Quan welcomed Mr Koizumi's words, but said that Japan had to do more to "face up to history".
"That President Koizumi expressed this attitude in this arena is welcome. We welcome it," Mr Kong told reporters.
"But to express it is one aspect. What's of much more importance is the action. You have to make it a reality."
Separately, China protested over Friday's visit by Japanese lawmakers to the Yasukuni shrine in Tokyo. The shrine honours the Japanese who died during World War II, including a number of war criminals.
"As Sino-Japanese relations are facing a serious situation, we express our strong dissatisfaction over the negative actions of some Japanese politicians who ignore the larger interests," the Chinese foreign ministry said in a statement quoted by AFP news agency.
In his speech, Mr Koizumi also repeated Japan's call for an overhaul of the UN Security Council and underscored Tokyo's qualifications as a potential permanent member.
"The United Nations, particularly the Security Council, needs to be reformed, so that the organisation reflects the realities of today's world."
The Japanese leader emphasised Tokyo's past contributions to development aid and repeated its commitment to increase them.
Japan's campaign for a permanent seat on the Security Council has been one of the factors fuelling the recent anti-Japanese protests in Chinese cities.
This Asia-Africa summit has special significance since it marks the 50th anniversary of the Bandung Conference - the first summit of Third World nations, in the shadow of the Cold War.
The summit intends to make a declaration of a strategic partnership to increase trade and investment as well as emphasise the importance of multilateral efforts in solving conflicts.
Developments so far have included:
- The highest level meeting between North and South Korean leaders for five years on the summit's sidelines. North Korean deputy Kim Yong-nam and South Korean Prime Minister Lee Hae-chan met for 10 minutes, Yonhap news agency reported.
A pledge by Japanese Prime Minister Junichiro Koizumi to double Japan's aid to Africa to $1.6bn over the next three years.
A statement by Indonesian President Susilo Bambang Yudhoyono that corruption threatened the development of Asia and Africa as much as war, HIV/Aids, and poverty.
- The first high-level meeting between India and Nepal since King Gyanendra became Nepal's monarch. Indian Foreign Minister Natwar Singh told the King to lift a state of emergency and free detainees held after his February coup.
Mr Yudhoyono said those taking part in the summit would make up for five decades of lost opportunities by establishing a new strategic partnership between Africa and Asia.
"Asia Africa is the missing link in the worldwide structure of interregional relations," he said in his opening remarks.
The meeting is being attended by a number of African presidents and prime ministers, including President Thabo Mbeki of South Africa, as well as the UN Secretary General, Kofi Annan.