The European Union (EU) signed the Association Agreements with Ukraine, Georgia and Moldova on Friday with the aim of deepening political and economic ties with the three former Soviet republics.
Ukrainian President Petro Poroshenko said at a ceremony in Brussels that Ukraine will use the opportunity to "modernize the country."
European Commission President Jose Manuel Barroso said: "This is indeed a historic day: for the three countries themselves, for the European Union and for the whole of Europe."
The deal is focused on supporting Ukraine in core reforms, economic recovery and growth, governance and sector cooperation, such as energy, transport and environment protection, the European Council said in a statement.
Poroshenko will discuss the violence in eastern Ukraine with Barroso and European Council President Herman Van Rompuy on the sidelines of an EU summit later on Friday.
Negotiations of this ambitious EU-Ukraine agreement were launched in March 2007. As a key part of the deal, the Deep and Comprehensive Free Trade Area (DCFTA) talks started in February 2008 following Ukraine's accession to the WTO.
Former Ukrainian President Viktor Yanukovych's refusal to sign the Association Agreement with the EU last November sparked massive demonstrations which led to the ouster of the pro-Russian leader in February.
The association agreement has two parts. Ukraine and the EU signed economic provisions of the pact on Friday, which removes tariffs on around 90 percent of the goods traded between the two sides. The political chapters were signed by both sides on March 21.
Russia on Friday warned of "grave consequences" for the Ukraine-EU agreement.
Russian Deputy Foreign Minister Grigory Karasin said the move could bring aggravations in the future.
Kremlin spokesman Dmitry Peskov said later that Russia would take measures to protect its economy as soon as it faced negative consequences of the deal.
However, Barroso defended the agreements, stressing that they were not against anyone.
"They are meant to add more momentum to our partners' established international relations, not to compete with - or intrude in - our partners' relations with any neighbour," he said after the signature ceremony.
Barroso also praised these agreements as a landmark in EU's Eastern Partnership policy, which will "enable our partner countries to drive reforms, to consolidate the rule of law and good governance; and to give an impetus to economic growth in the region by granting access to the world's largest internal market and by encouraging cooperation across a wide range of sectors.