"I'm not prepared to sign off on the delivery of additional aid for the Egyptian military," the Vermont Democrat said in a speech on the Senate floor, explaining why he would hold up the $650 million. "I'm not prepared to do that until we see convincing evidence the government is committed to the rule of law."
The Obama administration has been grappling for months with how to deal with Egypt, one of its most important allies in the Middle East. The Pentagon said last week it would deliver 10 Apache attack helicopters and $650 million to Egypt's military, relaxing a partial suspension of aid imposed after Egypt's military ousted President Mohamed Mursi on July 3 and violently suppressed protesters.
On Monday, an Egyptian court sentenced the leader of the outlawed Muslim Brotherhood and 682 supporters to death, intensifying a crackdown on the Islamist movement that could trigger protests and political violence ahead of an election next month.
Leahy said he would be watching the situation in Egypt with "growing dismay" even if he were not chairman of the State and Foreign Operations Subcommittee, denouncing "a sham trial lasting barely an hour."
"It's a flaunting of human rights by the Egyptian government. It's an appalling abuse of the justice system, which is fundamental to any democracy. Nobody, nobody, can justify this. It does not show democracy. It shows a dictatorship run amok. It is a total violation of human rights," Leahy said.
The Apaches are not subject to congressional approval.
Washington normally sends $1.5 billion in mostly military aid to Egypt each year, but a U.S. law - written by Leahy - bars funding governments brought to power via military coup.
The Obama administration wavered for months over whether to call events in Cairo a coup before cutting aid off in October to demonstrate unhappiness after the ouster of Mursi, who had emerged from the Muslim Brotherhood to become Egypt's first democratically elected leader.
The Pentagon, State Department and White House had no immediate response to Leahy's remarks.
However, U.S. Secretary of State John Kerry noted at an appearance on Tuesday with Egyptian Foreign Minister Nabil Fahmy that he would be discussing with Fahmy "disturbing decisions within the judicial process - the court system - that have raised serious challenges for all of us."
Fahmy, who was visiting Washington, said Egypt's judicial system was independent of the government and said he was confident due process was allowed in the courts.