Arriving in Riga to meet Latvian Prime Minister Maris Kučinskis straight from talks with UK Prime Minister Theresa May in London, both Tusk and Kučinskis stressed that the UK would only get post-'Brexit' access to the EU's prized single market if it continued to ensure existing freedoms to EU citizens.
"I know that Brexit is a particular concern for Latvia as so many of your citizens reside in the UK," Tusk said.
"Our goal is clear - to establish the best possible relations between the EU and the UK. It is obvious that once the negotiations begin, securing the rights of EU citizens in the UK will be a key objective for us and it's equally clear that our future relationship with the UK requires a balance of rights and obligations."
"Any access to the single market must be based on the four freedoms, including the freedom of movement," Tusk said.
The four freedoms in question are the free movement of people, goods, services and capital.
"But the negotiations cannot begin until the UK activates the process for withdrawal. Article 50 of the Treaty is very clear. In fact it is there to protect the interests of the countries remaining in the EU," Tusk added.
"I told Prime Minister May that I am convinced that it is in everyone's best interests that we start the negotiations soon to reduce and eventually end the uncertainty."
In what appeared to be a coordinated message ahead of a meeting of EU leaders (excluding the UK) in Bratislava on September 16 Kučinskis also stressed that without continued freedom of movement for EU citizens, the UK can forget about unrestrained market access.
"The new deal between the European Union and the United Kingdom must strike a balance between rights and obligations. Access to the single market can be granted only if the four basic freedoms are respected," he said.
The EU also needed to make "real progress in strengthening its external borders," at the Bratislava meeting, Kučinskis said.
Watch the statements below.