Ali Haider / EPA

An Egyptian woman casts her vote during the referendum for the Egyptian new constitution at the Egyptian consulate in the Gulf emirate of Dubai, United Arab Emirates, on Wednesday.

CAIRO -- Efforts to resolve Egypt's rapidly worsening political crisis suffered a blow on Wednesday when the army abruptly postponed "unity" talks that the opposition had minutes earlier said they would attend.

Confirmation that the secular, liberal opposition coalition would join the meeting after boycotting reconciliation talks hosted last week by Islamist President Mohammed Morsi had raised hopes of an end to street protests and deadly violence.

ANALYSIS: Egypt is rapidly approaching its own 'cliff'

The latest convulsion in Egypt's transition to democracy was brought on by a decree last month from Morsi in which he awarded himself sweeping powers to ram through a new constitution.

The constitution, to be voted on in a national referendum, is a necessary prelude to parliamentary elections due early next year.

Morsi's government forged ahead by starting voting in diplomatic missions abroad for expatriates on Wednesday. Hours after Egyptians began casting ballots overseas, the main opposition alliance called for a "No" vote rather than the boycott it had favored previously.

But the National Salvation Front's decision did not dispel the atmosphere of a nation in crisis, deeply polarized over the referendum.

The opposition still plans more protests and the country's judges are still on strike over Morsi's decree, which caused huge controversy and brought thousands of pro- and anti-government protesters onto the streets in the worst upheaval since the fall of Hosni Mubarak almost two years ago.

Sex mobs target Egypt's women

The unrest has so far claimed seven lives in clashes between the Islamist Muslim Brotherhood and the opposition. But the army has yet to use force to keep protesters away from the presidential palace, now ringed with tanks, barbed wire and concrete barricades.

There are also growing concerns about the already flailing economy a day after Egypt requested a postponement of a $4.8 billion IMF loan. Morsi suspended a package of tax hikes that had been part of a program to reduce Egypt's huge budget deficit for fear the measure would add to political tensions. 

Reuters and The Associated Press contributed to this report.

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Updated 10:10 a.m. ET: Miscommunication between a Spanish airline pilot and Dutch air traffic controllers caused a hijack scare on Wednesday that led the Netherlands to send F-16 fighters to intercept a plane. 

"There was never any danger. There was a lack of communication between the pilot and the tower and the airport has activated the security protocol," a spokeswoman for Spanish carrier Vueling said. 

The nature of the miscommunication was not immediately clear.

Dutch police said the security alert was triggered when radio contact with the plane was lost, Dutch news agency ANP reported. 

The plane, which was flying from Malaga in Spain to Amsterdam with about 180 passengers on board, was surrounded by Dutch security forces on landing at Amsterdam's Schiphol airport. Police then boarded and searched the aircraft.

The Dutch Defence Ministry had sent two F-16 fighters to intercept the airplane after suspecting a hijacking, a Dutch military police spokesman told Reuters. 

Robin Van Lonkhuijsen/Reuters

Passengers leave a Vueling plane which is parked at a field near Amsterdam Airport after a hijack scare Wednesday.

A passenger on board the plane said nothing unusual was happening, Dutch broadcaster NOS reported, quoting the passenger. 

"In fact nothing was going on. We had to fly a few rounds. We are now waiting in the plane, the doors are still closed. But there is no hijack," NOS quoted the person as saying. 

Airport staff said it was not clear when passengers would be allowed to disembark, and directed friends and relatives who were waiting at the arrivals hall to the information desk. 

Reuters contributed to this report

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