Congo's Kabila and Zimbabwe's Mugabe are close allies
The leaders also referred to separate Congolese peace negotiations in South Africa, urging the parties to reach an agreement in order to restore unity and stability to the country.
Of the five outside countries - Rwanda and Uganda on the side of the rebels and Zimbabwe, Angola and Namibia backing the Kinshasa government - only Namibia has pulled out all its troops.
The summit was called to end a deadlock in talks between the government, opposition parties, and rebels groups in South Africa.
These talks were intended to set up a transitional government which would oversee elections but little progress was made.
However, on the eve of the Lusaka summit, the government made an offer to let rebel groups share power, if they committed themselves to reunifying a country divided by three years of civil war.
Peace hopes
Opening the one-day summit, Zambian President Levy Mwanawasa told the warring factions that the peace process had reached a "critical" point.
"You, as Congolese parties, should bear in mind that the people of (DR) Congo are waiting with baited breath" for a successful conclusion of the peace talks, Mr Mwanawasa warned.
Presidents Robert Mugabe of Zimbabwe, Sam Nujoma of Namibia, Paul Kagame of Rwanda, Joseph Kabila of DR Congo and Thabo Mbeki of South Africa are in Lusaka.
The leaders of Angola and Uganda are being represented by senior government ministers.
Representatives from two of the three main Congolese rebel groups also attended the meeting.
Malnutrition
During the opening ceremony, the UN special envoy to DR Congo, Namanga Ngongi, said progress had been made towards peace, despite fighting in the south-eastern town of Moliro last month.
When UN peacekeepers visited the town on Tuesday, rival groups were no longer fighting.
Up to two million people have died in DR Congo, mostly from malnutrition and disease, since the war broke out in 1998 when rebels backed by Burundi, Rwanda and Uganda tried to topple the government of the late Congolese President Laurent Desire Kabila.
Peace hopes were revived after Mr Kabila's son, Joseph, succeeded his murdered father in January 2001.