Leaders from the 15-member ECOWAS regional bloc also said a high-level delegation would be sent to Ivory Coast as an "ultimate gesture" to Gbagbo in the hope that he could be persuaded to leave peacefully.
"In the event that Mr Gbagbo fails to heed the immutable demand of ECOWAS, the community will be left with no alternative but to take other measures, including the use of legitimate force, to achieve the goals of the Ivorian people," a final statement from the summit said.
The date referred to a previous emergency summit on the Ivory Coast crisis, when ECOWAS suspended the country from the organisation and called on Gbagbo to step down.
Pressure on Gbagbo has mounted since last month's presidential vote, with the United States searching for more UN troops and France offering him a final chance to step aside.
The summit was held after a UN body demanded a halt to "atrocities" in Ivory Coast and the Central Bank of West African States blocked Gbagbo's access to finances following the World Bank's earlier move to freeze loans.
Much of the world, including the United Nations, has recognised Ouattara as the winner of last month's elections, but Gbagbo has refused to budge in the face of escalating calls for him to leave.
The financial measures may make it difficult for Gbagbo to pay salaries for soldiers and others, and Ouattara addressed the military in a statement on Friday.
Ouattara read the statement in the Golf Hotel, the luxury Abidjan resort where his shadow government is holed up, protected by 800 UN peacekeepers who are in turn blockaded by Gbagbo's security forces.
"As commander in chief, I order the Defence and Security Forces to carry out their republican mission of protecting the population against the militias and foreign mercenaries that are spilling Ivorian blood," he said.
Both Ouattara's camp and UN human rights monitors accuse Gbagbo's FDS of involvement in large-scale human rights abuses, and there have been reports of them working with masked militia fighters and Liberian mercenaries.
"Violence is returning to our towns and our city neighbourhoods. Serious human rights violations are reported from all corners," Ouattara said.
"Under cover of the curfew, people have been been kidnapped and executed by elements of the Republican Guard and the Cecos, backed by foreign mercenaries and militiamen," he said, in his first public statement this month.
While commanders of the entire 17,000-strong FDS have pledged loyalty to Gbagbo, his most feared support comes from two elite squads, the 1,500-strong Guards and the 2,000 members of the Cecos police special forces unit.
|UN soldiers from Senegal stand guard in Abidjan, Ivory Coast.|
Ahead of Friday's summit, the United States had said it was talking with regional countries from ECOWAS about boosting the 9,000-strong UN mission in Ivory Coast.
French Foreign Minister Michele Alliot-Marie said Friday that Gbagbo could still step down honourably, but warned that time was growing short.
"Mr Gbagbo still has the possibility of leaving this situation with dignity by recognising what the results are and by handing over power," she told French radio.
"He has the right to a completely honourable exit... but the more time passes and the more things get out of control and there's violence, the more this possibility distances itself."