BRUSSELS — With Britain preparing to leave, Russia making threats and the United States giving it the cold shoulder, the European Union needs all the friends it can get.

But the bloc turned on itself on Tuesday when Jean-Claude Juncker, the president of the European Commission, tore into another branch of the body, the European Parliament.

“You are ridiculous,” Mr. Juncker said, after bashing the microphones in front of him.

Mr. Juncker, who leads the executive arm of the European Union, said he was miffed that only about 30 of the 751 members of the legislative body were present on Tuesday morning for a meeting at the legislature’s headquarters in Strasbourg, France.

Had Angela Merkel, the German chancellor, or Emmanuel Macron, the French president, been in attendance, he lamented, “it would have been a full house.”