The note tells officers to detain Assange "under all circumstances," The Independent reported. That includes his departure in a diplomatic vehicle.
A police spokesman said the document was notes taken by the officer carrying it at a briefing. The spokesman denied police would make any arrest violating diplomatic immunity but said Assange can be arrested for violating bail conditions.
Assange, an Australian national who faces extradition from Britain to Sweden, where he is under investigation for sex crimes, sought protection in the embassy. He could also be prosecuted in the United States on charges connected to the publication of diplomatic cables and other U.S. documents.
While 20 ambassadors from other South American countries went to the Ecuadorean embassy Friday in a show of support, the Organization of American States placed the Assange case on the agenda for a meeting in Washington.