Zambian President Levy Mwanawasa has called on the people of Zimbabwe to maintain peace at all costs.
Mr Mugabe used to be able to rely on support from his counterparts
He was speaking at the opening of a summit of southern African leaders at which Zimbabwe's economic and political crisis is expected to be discussed.
Countries in the region had all gone through difficult times, but had not resorted to violence, he said.
Observers say the remarks are highly unusual as African leaders are careful not to criticise one another openly.
The BBC's Peter Biles in the Zambian, capital, Lusaka, says Zimbabwe's President Robert Mugabe received a loud round of applause when he was introduced at the start of the Southern African Development Community (Sadc) summit.
But behind the scenes there is now clearly disquiet about the impact the Zimbabwe crisis could have on the economies of the neighbouring countries, he says.
Other nations taking part in the summit include Mozambique, Malawi, Botswana, Tanzania, Namibia, Lesotho and Swaziland.
"My advice to my brothers and sisters in Zimbabwe is: Maintain peace and stability at all costs," Mr Mwanawasa said, AFP news agency reports.
Mr Mwanawasa said leaders must be mindful of Zimbabwe's difficulties
"In the meantime, Sadc is there for you. This organisation is always ready to assist where it can to resolve the problems affecting member countries."
He said southern African leaders must be mindful of the difficulties that Zimbabweans were currently experiencing.
In March, Mr Mwanawasa likened the crisis in Zimbabwe to the sinking of the Titanic.
At the summit, South African President Thabo Mbeki is expected to report behind closed doors on his efforts to mediate between the Zimbabwean government and opposition.
Earlier, Tomaz Salomao, executive secretary of Sadc, told a news conference that the grouping had a range of options for Zimbabwe, including a "hard line", "quiet diplomacy" or a "different" method.
A senior Zambian official said Sadc had grown tired of the deepening political and economic crisis in Zimbabwe but he did not have a resolution, saying Mr Mbeki's progress report would determine a course of action.
But the BBC's Africa editor, Martin Plaut, says confronting Mr Mugabe goes against the grain of everything Sadc leaders hold dear.
All the leaders were given a warm welcome in Lusaka
The policy of apartheid drove South Africans into exile in the 1960's.
Then, under Ian Smith, Rhodesia declared independence in 1965.
The liberation movements found natural homes in Zambia, Mozambique, Angola and Tanzania - countries that formed the backbone of what were called the "frontline states."
Their armed wings fought shoulder-to-shoulder against the white regimes.
Rhodesian troops and South African special forces ranged across the region, killing their opponents as they went.
None of the region's leaders have forgotten this, and correspondents say Mr Mugabe used to be able to rely on the full support of his fellow leaders in southern Africa.
But the fall-out from Zimbabwe's political and economic crisis is now having a huge impact on the entire region, with increasing numbers of economic migrants fleeing Zimbabwe and settling in neighbouring countries.
Observers say there is a new mood of realism developing in the region, with Zimbabwe now seen as more than just a domestic problem.