Israel's Chief Rabbi praised Benedict's inter-faith outreach and wished him good health. The Archbishop of Canterbury, head of the Anglican Church, said he had learned of the Pope's decision with a heavy heart but complete understanding.
German Chancellor Angela Merkel said the Pope's decision must be respected if he feels he is too weak to carry out his duties. British Prime Minister David Cameron said: "He will be missed as a spiritual leader to millions."
The pontiff would step down from 1900 GMT on Feb 28, leaving the office vacant until a successor was chosen to Benedict who succeeded John Paul, one of history's most popular pontiffs, the spokesman said. Timeline: Benedict steps down as Pope
Elected to the papacy on April 19, 2005 when he was 78 - 20 years older than John Paul was when he was elected - Benedict ruled over a slower-paced, more cerebral and less impulsive Vatican.
Meek demeanour, steely intellect
But while conservatives cheered him for trying to reaffirm traditional Catholic identity, his critics accused him of turning back the clock on reforms by nearly half a century and hurting dialogue with Muslims, Jews and other Christians.
Under the German's meek demeanour lay a steely intellect ready to dissect theological works for their dogmatic purity and debate fiercely against dissenters. Pope Benedict: diehard traditionalist who resigned his post
After appearing uncomfortable in the limelight at the start, he began feeling at home with his new job and showed that he intended to be Pope in his way.
Despite great reverence for his charismatic, globe-trotting predecessor -- whom he put on the fast track to sainthood and whom he beatified in 2011 -- aides said he was determined not to change his quiet manner to imitate John Paul's style.
A quiet, professorial type who relaxed by playing the piano, he managed to show the world the gentle side of the man who was the Vatican's chief doctrinal enforcer for nearly a quarter of a century.
The first German pope for some 1,000 years and the second non-Italian in a row, he travelled regularly, making about four foreign trips a year, but never managed to draw the oceanic crowds of his predecessor.
The child abuse scandals hounded most of his papacy. He ordered an official inquiry into abuse in Ireland, which led to the resignation of several bishops.
String of scandals
Scandal from a source much closer to home hit in 2012 when the pontiff's butler, responsible for dressing him and bringing him meals, was found to be the source of leaked documents alleging corruption in the Vatican's business dealings, causing an international furore.
He confronted his own country's past when he visited the Nazi death camp at Auschwitz.
Calling himself "a son of Germany", he prayed and asked why God was silent when 1.5 million victims, most of them Jews, died there during World War Two.
Pope Benedict XVI greets people in front of a huge Jesus Christ portrait in Krakow in this May 27, 2006 file photo. Reuters