An assault force of U.S. marines and Army rangers and soldiers from seven small Caribbean nations invaded Grenada before dawn and easily seized both airfields in the small island country. But the invaders' advance was reportedly slowed in the afternoon by heavy fire in the capital. (Page A1, Column 5.) President Reagan announced he had ordered the invasion of Grenada by nearly 1,900 Marines and Army airborne troops to protect United States citizens and to help restore democratic institutions in the Caribbean island. The casualties were said to be low. Mr. Reagan said the operation was launched in response to an urgent request from countries close to Grenada, some of which provided 300 troops for the assault. The United States, he said, ''had no choice but to act strongly'' to oppose ''a brutal gang of leftist thugs'' that violently seized power on Oct. 12 and later executed the ousted Prime Minister and other Cabinet members. (A1:6.)

The Soviet press assailed the invasion as an ''act of undisguised banditry and international terrorism'' designed to ''subordinate Grenada to U.S. neocolonialist rule.'' (A17:1-2.) Britain distanced itself from the invasion. Prime Minister Margaret Thatcher said the British Government had urged President Reagan to reconsider the plan after learning of it Monday and that no British ships or forces had taken part. (A1:4.) President Reagan was considering a secret request from Caribbean countries to join in an invasion of Grenada while he was playing golf in Augusta Saturday. The national security meetings that followed were ostensibly devoted entirely to the Lebanon crisis, but yesterday it became clear that they also advanced the decison to take part in the invasion. (A1:5-6.) Most Congressmen were shocked and astonished at word that American troops had invaded Grenada, and they split sharply over the wisdom of the policy. Conservatives from both parties hailed the move. Speaker Thomas P. O'Neill Jr. said President Reagan deserved bipartisan support in a time of crisis. (A22:3-6.) Bombings in Beirut The highest state of alert was ordered for the Marine contingent in Beirut as Gen. Paul X. Kelley, the United States Marine commandant, arrived in the capital. The alert was ordered after the marines received an intelligence warning from the Lebanese Army saying that three vehicles believed to be carrying high explosives were circling the area and might strike at the Marine compound. Officials announced that the number of American military personnel killed in Sunday's bombing had risen to 216, with 20 to 30 bodies still believed buried in the rubble of the headquarters building. Also, a marine officer said that two long, thick pipes that had been placed as barriers in front of the entryway of the building to protect against car-bomb attacks had been removed several days before Sunday's attack. (A1:1.) The adequacy of security measures to protect American marines in Beirut was questioned by some members of Congress. (A6:1-2.) More families received word that a loved one had been among the more than 200 American servicemen killed in the terrorist attack in Beirut. The families reacted variously with tears, frustration, pride and outrage. Their views of United States involvement in Lebanon ranged from quiet support to unqualified hostility. Some families, overcome with grief, remained in seclusion. (A7:1-4.) International Global curbs on press freedom were urged by Moscow at a Unesco conference in Paris as part of a ''new world information order'' that would give Governments control over the flow of news and information. (A11:1-3.) National President Reagan dismissed the three Democratic members of the six-member United States Commission on Civil Rights who have sharply criticized his policies toward blacks, women and Hispanic Americans over the last two years. The dismissals came as a surprise to the commissioners and to Senators who had been trying to negotiate a bipartisan compromise on the agency. (A24:1-3.) Efforts to cut the budget deficit were renewed by the House, which approved $10.3 billion in spending reductions over three years. The bill, which passed on a voice vote, also includes a provision to require the President to hold a bipartisan, top-level domestic economic meeting to deal with high budget deficits. (B7:1-2.) Metropolitan Assertions of police brutality were rebutted by Police Commissioner Robert J. McGuire. He offered a detailed rebuttal to 98 allegations of racially-motivated brutality that were raised last month at a Congressional hearing in Harlem. Mr. McGuire said that in each incident ''the officers involved acted properly and professionally.'' (B3:3-6.) The two major candidates for Westchester County Executive disagreed on a wide range of issues, including the availability of low-cost electrical power and Westchester's financial needs. The two opponents, Andrew P. O'Rourke and Ogden R. Reid, debated for 90 minutes at The New York Times. (B1:2.) Page D1