Germany's main opposition leader vowed on Thursday to impose immediate border controls if he becomes chancellor, as expected, after elections next month, a day after an Afghan asylum seeker was arrested for a deadly knife attack targeting children.

Two people, including a two-year-old boy, were killed and two others were severely injured in a stabbing attack in Bavaria on Wednesday, German police said. Police said a suspect was detained in the knife attack, which occurred in a park in the southern German town of Aschaffenburg.