The United States has led a chorus of international concern about Egypt's crackdown on demonstrators, publicly condemning the violence that resulted in the worst loss of life on a single day since the overthrow of President Mohamed Morsi last month.

The White House said "the world is watching" after a day on which at least 278 people were killed. But there was still no sign that the US was prepared to characterise Morsi's removal by the army as a coup – which would trigger an automatic congressional ban on $1.3bn in annual aid to the powerful Egyptian military.

"The violence will only make it more difficult to move Egypt forward on a path to lasting stability and democracy and runs directly counter to the pledges by the interim government to pursue reconciliation," said spokesman Josh Earnest.

Lasting stability appeared further away than ever on Wednesday evening after the military declared a month-long state of emergency and the liberal Mohamed ElBaradei resigned as vice-president in the military-backed interim government.

The first night of the curfew imposed in Cairo was enforced by police and soldiers, aided by self-styled "popular committees" of civilian vigilantes armed with clubs and machetes, Reuters reported. They searched cars and checked identity cards of people passing through makeshift checkpoints made of tyres and concrete blocks.

Despite the lockdown, hundreds of Morsi supporters tried to gather at El Iman mosque in the neighbourhood of Nasr City in an attempt to start a new sit-in. The BBC broadcast footage of numerous fires burning in the city, but no large-scale conflict was reported overnight.

The National Alliance to Support Legitimacy had called on "all Egyptian people" to take to the streets "to stop the massacre" after police attacked its two sit-ins in Cairo's Nahda and Rabaa al-Adawiya squares early on Wednesday.

The alliance is an Islamist grouping led by the Muslim Brotherhood, which has been demanding Morsi's reinstatement as president since he was ousted by the army. Morsi supporters called for further nationwide protests.

Fatalities on Wednesday included the 17-year-old daughter of Mohamed Beltagi, a Brotherhood leader. Three other senior figures were reportedly detained in what appeared to be the start of a wide-ranging crackdown on the Islamist movement. Egypt's health ministry said that 235 civilians had been killed and 1,400 injured, while Interior minister Mohammed Ibrahim said 43 policemen had died. A statement issued by the Egypt Anti Coup Alliance said "more than 2,000" had been killed.

Trouble also spread beyond Cairo, with reports of a church set on fire in Sohag, 250 miles south of the capital. Ten people were killed in Alexandria on the Mediterranean coast.

ElBaradei's resignation statement underlined the dilemma faced by liberal and secular supporters of the revolution that toppled Hosni Mubarak in February 2011. "It has become difficult for me to hold responsibility for decisions that I do not agree with, whose consequences I fear," ElBaradei said as a curfew was imposed across the country.

"I cannot be responsible for one drop of blood in front of God, and then in front of my conscience, especially with my faith that we could have avoided it." The Nobel laureate said that those who incited "violence and terrorism" – language the government has used to describe the Brotherhood – would only benefit from the turmoil.

Britain's foreign secretary, William Hague, said he was "deeply concerned" at the escalating violence and unrest. "I am disappointed that compromise has not been possible. I condemn the use of force in clearing protests and call on the security forces to act with restraint."

Baroness Ashton, the EU's foreign policy chief, who met Morsi in his place of detention earlier this month, said in a statement: "Confrontation and violence is not the way forward to resolve key political issues. I deplore the loss of lives, injuries and destruction in Cairo and other places in Egypt."

The UN secretary general, Ban Ki-moon, condemned the violence and urged an effort at "inclusive reconciliation". France and Germany also called for dialogue.

The strongest language came from Turkey, whose government has been a firm supporter of the Egyptian Brotherhood. It urged the UN security council and the Arab League to act quickly to stop a "massacre". Iran warned of the risk of civil war. Rachid Ghannouchi, president of Tunisia's governing moderate Islamist party Ennahda, called the crackdown an "abject crime". He expressed solidarity with the pro-Morsi backers' bid to "recover their freedom and oppose the coup d'etat".

Analysts said that the response from Washington fitted a pattern of weak statements that had allowed the Egyptian military to act with impunity. "[The] US had several chances to demonstrate [that] its threats to suspend aid were credible, but each time backed down," tweeted the Brookings Institution expert Shadi Hamid. "That policy has a price." Hamid also told al-Jazeera TV: "Clearing all the sit-ins without addressing fundamental political issues won't stop the clashes."

Marc Lynch commented in Foreign Policy: "It's time for Washington to stop pretending. Its efforts to maintain its lines of communication with the Egyptian military, quietly mediate the crisis and help lay the groundwork for some new, democratic political process have utterly failed. Egypt's new military regime, and a sizable and vocal portion of the Egyptian population, have made it very clear that they just want the United States to leave it alone.

For once, Washington should give them their wish. As long as Egypt remains on its current path, the Obama administration should suspend all aid, keep the embassy in Cairo closed, and refrain from treating the military regime as a legitimate government."