The actual Brexit talks will not start for around three or possibly more weeks after the summit.
Tusk's announcement came a day after London said Prime Minister Theresa May would trigger the two-year process for Britain's departure on March 29.
"In view of what was announced in London yesterday, I would like to inform you that I will call a European Council on Saturday the 29th of April to adopt the guidelines for the Brexit talks," Tusk told a press conference in Brussels.
The European Council president has said he will issue the draft guidelines for the 27 leaders within 48 hours of May triggering Article 50 -- the divorce clause in the EU's treaties -- next week.
"I personally wish the UK hadn't chosen to leave the EU but the majority of British voters decided otherwise. Therefore we must do everything we can to make the process of divorce the least painful for the EU," he said, standing alongside visiting Japanese Prime Minister Shinzo Abe.
"Our main priority for the negotiations must be to create as much certainty and clarity as possible for all citizens, companies and member states that will be negatively affected by Brexit."
Tusk said the EU would also seek to lessen the impact on "our important partners and friends around the world, like Japan."
The actual Brexit talks will not start for around three or possibly more weeks after the summit, as leaders have to approve more detailed negotiating lines and give a mandate to European Commission negotiator Michel Barnier, EU officials said.
Time will now be limited for Britain to negotiate the terms of its departure and a future trade deal by the deadline of March 2019.
Barnier has said a draft deal must be on the table by October 2018 if there is to be time to get it ratified by national assemblies and the European Parliament.