SAN FRANCISCO—The European Commission is ramping up pressure on tech companies to more aggressively use automated filtering to scrub “illegal” content from the internet, a move that is drawing criticism from some lawyers and free speech activists in Silicon Valley.

In a communication issued Sept. 28, titled “Tackling Illegal Content Online,” the commission said it “strongly encourages online platforms to use voluntary, proactive measures” to pull down illegal content and to pour more money into “automatic detection technologies.”