The Obama administration notified Congress on Friday that it intends to give Egypt’s new government an emergency cash infusion of $450 million, but the move immediately encountered resistance from lawmakers wary of foreign aid in general and of Egypt’s new course under the leadership of the Muslim Brotherhood.

The aid is part of $1 billion in assistance that the Obama administration has pledged to Egypt to bolster its economy and its transition to democracy following the overthrow last year of Hosni Mubarak, the country’s autocratic president.

Its fate, however, was clouded because of concerns over some of the new government’s policies and its handling of anti-American protests two weeks ago that overwhelmed the American Embassy in Cairo.

The United States Agency for International Development notified Congress of the cash infusion on Friday morning, prompting renewed protests over the foreign aid and the administration’s handling of crises spreading across the Islamic world.

An influential Republican lawmaker, Representative Kay Granger of Texas, immediately announced that she would act to block the assistance because it came at a point when the American relationship with Egypt “has never been under more scrutiny.”

“I am not convinced of the urgent need for this assistance and I cannot support it at this time,” Ms. Granger said in a statement.

State Department and White House officials have shuttled to and from Cairo to negotiate the assistance. Although much of the $1 billion in aid previously announced was intended to relieve Egypt’s debts to the United States, the administration decided to provide cash instead — including $190 million immediately — because the country’s budget crisis has become acute.

Secretary of State Hillary Rodham Clinton, speaking at a meeting in New York on the sidelines of the annual United Nations General Assembly, said the world needed to do more to support the governments that have emerged from the popular uprisings of the Arab Spring.

“The recent riots and protests throughout the region have brought the challenge of transition into sharp relief,” Mrs. Clinton said, without mentioning the assistance to Egypt specifically. “Extremists are clearly determined to hijack these wars and revolutions to further their agendas and ideology, so our partnership must empower those who would see their nations emerge as true democracies.”