In particular, the politician criticized Merkel's approach toward Turkey and called for a harder stance in relations with Ankara.
"We are witnessing a Turkish president who breaks international rules," Schulz said. "German politicians must react to this unacceptable approach."
The SPD candidate accused the chancellor of being too hesitant about Erdogan's provocations.
"How long are we going to sit idly, while Mr. Erdogan fools us around?" Schulz wondered.
However, Schulz is confident that he could still win by appealing to those who have not yet decided on their preferences. Research published last week showed that 46 percent of voters are still undecided.
"I want to reach those undecided people," Schulz said. "And if I can do that, then I can win the parliamentary elections."
On September 24, German citizens will head to the polls to elect their new government. Following the election, the Bundestag will elect the new chancellor, with Angela Merkel and Martin Schulz being the main candidates for the post.