Tusk tweeted a photograph of himself and other European and Asian leaders standing in tribute to the Nice victims at the Asia-Europe meeting here in the Mongolian capital.
"European and Asian leaders pay respect to victims of Nice attack. We stand united against violence and hatred," Tusk tweeted.
He said the attack was a "tragic paradox" that the victims of the attack in Nice were celebrating "liberty, equality and fraternity" - France's motto - on the country's national day.
Li made the remarks while attending the two-day 11th ASEM Summit held in Ulan Bator in Mongolia.
Two Chinese nationals were injured in the deadly attack in the French resort of Nice last night, which killed 77 people, the state-run Xinhua news agency reported.
The attack comes eight months after Islamic State terrorists attacked Paris nightspots, killing 130 people and dealing a hard blow to tourism in one of the world's top destinations.