WikiLeaks claims that items from the time Julian Assange lives at Ecuador's embassy in London will be handed over to US prosecutors.
Ecuador officials are said to be heading to London to allow US prosecutors to "self-help" with items including legal documents, medical records and electronic equipment.
According to WikiLeaks, United Nations officials and Assange's lawyers were not allowed to attend, calling them the puppet show ".
Lawyers say it was an act of illegally seizing property and was requested by the US authorities.
Two of Assange's manuscripts are said to be among them.
Kristinn Hrafnsson, editor-in-chief of WikiLeaks, said: "On Monday, Ecuador will perform a puppet show for their host in Washington at the Ecuador embassy in London, just in time to expand their extradition cases by the UK deadline of June 14.
"The Trump administration is inducing its allies to act like Wild West.
Baltasar Garzon, international legal cooperation Corporation
The coordinator who defended Assange and WikiLeaks said: "Ecuador continues to search and seize property, documents, which is very worrying and Ecuador arbitrarily confiscated information and other materials belonging to Julian Assange's defense, in this way, these can be handed over to the agents of the United States for political persecution against him.
"This is an unprecedented attack on defense rights, freedom of speech and access to information that reveals massive human rights violations and corruption.
We call on international protection agencies to intervene to stop such persecution.
"Earlier this week, Ecuador said it was awaiting the results of an investigation into his belongings and claimed that he had repeatedly violated his asylum conditions by attempting to use the embassy to engage in espionage.
The Minister for Foreign Affairs of Ecuador, José Valencia, said: "For us, the allegation that the Ecuadorian embassy in London is spying on Assange is clearly unfounded.
"I want everyone to watch a documentary shot by friends of the Guardian reporter Laura Portas and Assange.
The documentary, called risk, aired on Netflix.
"In 58 minutes, you saw Assange at the Ecuador embassy in London, and his laptop was connected to the Internet via the cable from our embassy.
You can clearly see him sitting there and you can see the screen.
"What is shown on the security camera screen of the embassy, he said that these cameras are monitoring him.
He seems to be operating them all the time with a laptop.
"Assange served 50 in prison-
Sentenced to one week in prison for breach of bail
Seven years later, the state decided to stop providing him asylum in London, and he was dragged out of the Ecuadorian embassy.
He already faces an extradition request from the United States, and Sweden may file an extradition request after the prosecutor decides to re-investigate him.