They say the envelopes were found at the house of Abu Hassan, on trial in the southern town of Zinjibar with four other defendants for the kidnap and killing of Western tourists in December. Three Britons and an Australian died in the shoot-out with government forces and 12 hostages survived.
The prosecutors producedmanuals on guerrilla tactics and the use of AK47 assault rifles, and light armour, military maps, and audio cassettes allegedly of propaganda by Islamic fundamentalists, including Abu Hamza, all said to have been found at the house. There were also pictures of Osama bin Laden, the Saudi dissident blamed for the bombing of US targets in Kenya, Tanzania, and Saudi Arabia.
The envelopes were addressed to Uthman al-Saedi at a post office box in the Yemeni Sana'a. The number had been obliterated. The envelopes also bore stamps with the identification code London WC229. The prosecution did not disclose the contents.
The Yemeni authorities say Abu Hamza is linked to the kidnappers and the eight Britons on trial in Aden on terrorist charges.
They believe he sent the Britons to inflict bombings in Aden, and after their arrest Abu Hassan took the tourists hostage to bargain for their release. Abu Hamza has denied the Yemeni charges.
The prosecution had earlier produced a satellite telephone which it said was sent to the kidnappers by Abu Hamza, delivered by one of the Britons on trial in Aden, and used by Abu Hassan to contact Abu Hamza.Yemen has not formally asked for Abu Hamza's extradition.
Abu Hassan has admitted the charges but denied receiving funding from abroad. He said he had planned the bombings of British and American targets in Aden, and added: "I am not ready to convict others."
Two of his co-defendants deny the charges and two admit kidnap. The trial continues on Sunday.
Nick Childs is a BBC World