Clashes erupted on the streets of Cairo on Monday not long after a visiting U.S. diplomat hailed what he called a “second chance” for Egyptian democracy after the ouster of the country’s elected president this month.

Deputy Secretary of State William J. Burns, the highest-level U.S. official to visit Cairo since President Mohamed Morsi was removed by the military on July 3, signaled Washington’s readiness to stand with Egypt’s new leaders even as Morsi supporters were gathering for fresh demonstrations to demand his reinstatement.

“The United States is firmly committed to helping Egypt succeed in this second chance to realize the promise of the revolution,” Burns told a small group of reporters after a day spent meeting with members of the new interim government, including Gen. Abdel Fatah al-Sissi, the army chief and defense minister.

“I am not naive. I know that many Egyptians have doubts about the United States, and I know that there will be nothing neat or easy about the road ahead,” he added.

Burns’s language seemed to underline a shift by the Obama administration in the past two weeks, from warning against unseating a democratically elected president to throwing its weight behind the backers of the coup.

Underscoring the challenge ahead for the United States, Burns was rebuffed by representatives of both the group that led the popular uprising against Morsi and the ultraconservative Islamist party that could benefit from the ouster.

The Tamarod, or rebel, movement that instigated the anti-Morsi demonstrations declined an invitation to attend a round­table discussion with Burns and U.S. Ambassador Anne Patterson at a top Cairo hotel, accusing the United States of supporting Israel and backing the Morsi-allied Muslim Brotherhood, according to a statement on the group’s Web site.

“I would like to ask them, what business of yours is Egypt,” Mahmoud Badr, one of Tamarod’s founders, asked in a posting on his Facebook page.

The Salafist Nour party also refused to meet with Burns, Egyptian media reported. The official Middle East News Agency quoted an unnamed Nour member as saying that the party rejected “American interference in Egyptian affairs.” The Salafists joined the liberal Tamarod movement and other anti-Islamist opposition groups in calls for Morsi to step down.

The Obama administration’s refusal to label Morsi’s ouster a coup also has offended the Brotherhood, which has accused Washington of complicity in his downfall. Brotherhood spokesman Gehad el-Haddad said the organization had not been invited to meet Burns and had not sought an invitation.

Burns urged a “serious and substantive dialogue” between the pro and anti-Morsi factions to restore calm in Egypt, even as tensions between the two camps were escalating anew.

Just hours after he spoke, Morsi supporters surged into central Cairo, blocking roads and highways and occupying Ramses Square, a major intersection. Police fired tear gas to disperse them, and anti-Morsi youths hurled molotov cocktails at them. The protesters set fire to tires to defend their positions, filling the city center with clouds of smoke and gas.

Similar scenes were seen in other Egyptian cities, including Assiut to the south and Alexandria in the north as the Brotherhood escalated a campaign of civil disobedience aimed at forcing the authorities to reinstate Morsi.