The Asia-Europe Meeting (ASEM) in the Mongolian capital began with a moment's silence Friday following the attacks in Nice.
"It's a sad day for France, Europe and all of us here in Mongolia," EU President Donald Tusk said.
"We are united with the French people and government in their fight against violence and terror."
In his opening remarks the host, Mongolian President Tsakhia Elbegdorj, spoke of "very sad news from France."
He called on the 34 heads of government and state present from 51 countries in Europe and Asia to observe a minute's silence for the victims.
Robert Fico, prime minister of Slovakia, which currently holds the rotating presidency of the Council of the EU, told the meeting that their discussions should include reflection on what happened in Nice.
"The fight against terrorism is among the highest priorities of our governments. We condemn terrorist attacks in every form, wherever and whenever they occur," Fico said.
The summit has already been overshadowed by the dispute over China's territorial claims in the South China Sea.
The Permanent Court of Arbitration in The Hague on Tuesday upheld a complaint brought by the Philippines about contested islets in the South China Sea, which holds key shipping lanes and is believed to be rich in mineral and marine resources.
China refused to take part in the arbitration and said the verdict was null and void.
ASEM was established in 1996 to deepen relations between Asia and Europe.