A group of Israelis describing themselves as reservists in elite military and intelligence units said they would not turn up for some duties from Sunday, escalating protests at the hard-right government's planned judicial overhaul.
Members of Prime Minister Benjamin Netanyahu's coalition, which wields a Knesset majority, say they want bills the would limit Supreme Court authorities written into law by April 2.
The plan has stirred concern for Israel's democratic health at home and abroad. As ratification nears, demonstrations have spiraled, the shekel has slipped and fears have been voiced by national security veterans who usually shy from public exposure.
In a letter circulated to the Israeli media, 450 protesters describing themselves as volunteer reservists from military special forces and another 200 as volunteer reservist offensive cyber operators, including from the Mossad and Shin Bet intelligence agencies, said they were now refusing call-ups.
Reuters could not verify the signatories' identities and the secrecy around the units they said they belong to also made it difficult to assess the protest's potential impact.
"We have no contract with a dictator. We would be happy to volunteer when the democracy is safeguarded," the letter said.
The military declined comment. Representatives for Mossad and Shin Bet did not immediately respond to queries by Reuters.
Netanyahu calls the judicial overhaul a restoration of balance between the branches of government. Critics see a gambit by the prime minister - who is under trial on corruption charges that he denies - to subordinate the courts to the executive.
On Sunday, a Knesset review committee was due to discuss, before final voting sessions in the plenum, a bill that would give the coalition more control over appointments to the bench.
That, critics say, could foster corruption and imperil judicial independence key to Israel's economic strength and defenses against attempts to isolate it internationally.
Netanyahu has condemned the protests' reach into the military ranks as an attempt to subvert an institution meant to be above politics. Such misgivings have been voiced by some opposition leaders, while others say an authoritarian tilt in government would throw the idea of national duty into question.
"When a country stands on the threshold of dictatorship, we are likely to see a break-down of the security agencies," former Shin Bet director Nadav Argaman told Channel 12 TV. "It is extraordinarily terrifying."
A man describing himself as a military intelligence captain taking part in Sunday's reservist protest told Kan radio that he and other signatories were deemed volunteers in part because their time commitments exceeded normal quotas for reservists.
Signalling that the protest would be suspended in the event of a compulsory wartime call-up, he said: "We are not calling for refusing orders. We calling for a halt to the volunteering."
Most Israelis are conscripted into the military for between two and three years. Some continue to do reserve duties into middle-age. While reservists have helped Israel prevail in previous wars, more recently it has relied on regular forces.
But some units consider reservists especially valuable given their maturity and accrued skills. An air force pilot taking part in the protests told Channel 12 TV that as many as 60% of crews sent on bombing sorties in Syria are volunteer reservists.