President Mugabe's presence was not announced in the programme
Zimbabwe's three leading figures have condemned international sanctions on the country at a World Economic Forum conference in Tanzania.
In a rare show of unity, President Robert Mugabe, Prime Minister Morgan Tsvangirai and his deputy Arthur Mutambara appealed for investment.
Only Mr Tsvangirai had been expected to represent Zimbabwe at the forum in Dar es Salaam.
But President Mugabe and Mr Mutambara made a surprise appearance.
Despite the history of conflict, the three men put on a civil front, though Mr Mutambara did not resist a rather barbed reference to having gone to a previous World Economic Forum from a prison cell, says the BBC's Andrew Walker, who is in Dar es Salaam.
The Zimbabwean leaders were speaking to an audience with a large business contingent.
EU: 2002 to present
Assets freeze and travel ban on some Mugabe allies, arms-sale ban
US: 2003 to present
Trade ban against 250 Zimbabwean individuals and 17 companies
Canada, Australia and UK among nations to have imposed their own targeted sanctions
Sources: EU, Reuters, US treasury, UK Foreign Office
Several of them welcomed the show of unity and the more stable economic environment that Zimbabwe now has, our correspondent says.
But there were also some concerns expressed about how durable the improvement will be, he adds.
Mr Tsvangirai said his country no longer represented a risk to investors.
"The political crisis does no longer exist.
"The country is making progress and it's time that investors started looking at Zimbabwe from a different perspective," he was quoted by Reuters news agency as saying.
Mr Mugabe reiterated the call for an end to sanctions.
"Why the sanctions should be imposed on us we don't understand to tell you the truth, and this from Europe and America and not from the rest of the world," he said.
If the three leaders were here to give an invitation to investors, their joint appearance may have helped a little, our correspondent says.