European Council President Donald Tusk accused Russia of "aggressive and bullying tactics" towards its ex-Soviet neighbours. Ukraine and Georgia are among the EU's eastern partners.
But Mr Tusk said the partnership was a "long haul", not aimed against Russia.
It is overshadowed by Russia's role in the conflict in eastern Ukraine.
Mr Tusk said that if Russia was "a bit softer, more charming [...] perhaps it would not have to compensate its shortcomings by destructive aggressive and bullying tactics against its neighbours."
He said that the bloc was determined to support its poorer neighbours despite Russian pressure: "The European Union stays the course despite the intimidation, the aggression, even the war, of the last year," he said.
As the two-day Riga summit got under way, Georgia and Ukraine were conducting military training drills with US troops.
Russia accuses the US and Nato of striving to pull its former Soviet partners into the Western camp, to Russia's detriment.
It is exerting pressure on ex-Soviet states to join a Moscow-led "Eurasian Union".
A far-reaching EU-Ukraine association agreement has angered Russia.
At the summit the EU is expected to approve €1.8bn (£1.3bn; $2bn) for Ukraine - the last instalment of a €3.4bn aid programme.
There is much alarm in Latvia and its Baltic neighbours Estonia and Lithuania over Russia's annexation of Crimea and support for pro-Russian rebels in eastern Ukraine.
The three Baltic states were under Russian domination during decades of communism.