In a case that has raised issues about press freedom and German relations with Egypt under President Abdel Fattah al-Sisi, Mansour was arrested yesterday at a Berlin airport having been accused by Cairo of committing "several crimes".
"He is in police custody," a spokesman for the Berlin public prosecutor's office, Martin Steltner, told AFP today.
"The Berlin prosecutor's office is examining the legal assistance request" from Egypt, he added.
Al-Jazeera said on its website that an Egyptian court had sentenced Mansour in absentia in 2014 to 15 years in prison, for "torturing a lawyer in 2011 on Tahrir Square" in Cairo, epicentre of an anti-regime uprising that brought down former president Hosni Mubarak.
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"Mansour has rejected these absurd accusations," the network said.
Mansour tweeted angrily in detention yesterday: "The question now is how have the German government and Interpol become tools in the hands of a bloodthirsty regime in Egypt that came to power through a coup, and is led by the terrorist (President) Abdel Fattah al-Sisi."
The journalist has rejected charges against him as "false" and urged Berlin against colluding with Cairo, in a video aired by the broadcaster today.
"This case is false," Ahmed Mansour said in the video message he recorded in police detention in the German capital.
"The coup regime in Egypt is too weak to drag a state like Germany and the EU into its dirty game against Egyptians," said Mansour, referring to the Egyptian military's ouster of Islamist president Mohamed Morsi in July 2013.
Mansour, who hosts a popular news programme, recently interviewed Abu Mohamed al-Jolani, the chief of Al-Qaeda's Syria affiliate, Al-Nusra Front.
Egypt's Sisi visited Germany on June 3, as prominent rights groups urged Chancellor Angela Merkel to press him to end "the gravest human rights crisis in Egypt in decades."
Merkel in a joint press conference with Sisi voiced criticism of Egypt's use of the death penalty and record on religious freedom, but pledged closer economic ties with its partner in the fight against Islamic extremism.