The 30-year-old, who lost his world heavyweight title fight to ex-champion Vitali Klitscho in 2012 in a one-sided bout, was shot late on Tuesday night, and the gunman is still at large.
Police in Essen confirmed to SID, an AFP subsidiary, that they have launched an attempted murder investigation to catch the offender and armed officers stormed a flat in Essen on Wednesday morning, but no arrests were made.
Charr underwent emergency surgery after being rushed to hospital in the west German city, but his life is not in danger and the reason for the attack is not known.
Charr, nicknamed "Diamond Boy", lost his last bout when he was knocked out by Latvia's Mairis Breidis in Moscow last month having earned a unanimous points win over Australia's Alex Leapai in May.
This is not the first time Charr, who is of German-Lebanese origin, has got into trouble outside the ring after being involved in a knife fight in August 2006, after throwing raw eggs at passers-by on Berlin's busy Kurfuerstendamm street.
Charr faced charges of attempted murder, but was acquitted due to a lack of evidence.
He has a record of 16 knock-outs in 28 wins from 32 bouts with four defeats, three of which were by knock-out.
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