Undated (AP) _ Millions of Americans welcomed the new year Friday in outdoor chill and indoor warmth with parties, colorful parades and college football bowl games. Thirty members of a Colorado club even climbed 14,000-foot-high Pike's Peak in howling winds and set off fireworks visible 60 miles away in Denver.

Crowds along the 5 1/2 -mile route of the Rose Parade in Pasadena, Calif., estimated by police at ''just under a million,'' watched the 60-float procession festooned with nearly 15 million flowers, 22 bands and 275 horses. Four spectators, a parade mechanic and another person were injured in parade accidents.

In downtown Dallas, thousands of spectators braved temperatures in the low 30s to watch the Cotton Bowl parade.

On the East Coast, Philadelphia celebrated in traditional fashion with the Mummers Parade featuring comics and string bands strutting in sequins and feathers. Early morning rain caused an hourlong delay.

''If you don't march on New Year's Day, it's not the same,'' said Tim McShea, 32, who was wearing a chicken suit and orange claws. Last year the parade was postponed twice because of inclement weather.

President Reagan and Mikhail Gorbachev exchanged optimistic New Year's greetings, each expressing hope that they will reach a second arms control treaty in the first half of the new year.

In Los Angeles, random gunfire from New Year's Eve partygoers killed at least two men and officers seized a small arsenal while arresting seven shooters.

Reports of shots heralding the end of 1987 were widespread in Southern California, but law enforcement agencies said the practice, which had become a kind of ritual in recent years, appeared to have fallen off slightly.

In Detroit, police said no injuries were reported as a result of random gunshots.

''I had planned to go to my basement, but this year the shooting started before midnight so I hit the floor and sat away from any openings,'' said Jeffery Waters, who said a bullet came through the roof of her sun porch.

Despite chilly weather, large crowds gathered in New York, Boston and Washington, D.C., to ring in the new year.

An estimated 350,000 people jammed into New York's Times Square, raucously counting down the final seconds as a glowing ball in the shape of a ''Big Apple'' descended a flagpole to begin 1988.

In Boston, some 500,000 people crowded into the area around the Common for the city's 12th annual First Night celebration, highlighted by a parade, outdoor art events, performances and a 15-minute burst of midnight fireworks.

Tens of thousands filled The Old Post Office in Washington for the fifth annual New Year's party in the nation's capital. At midnight, a lighted prototype of the Postal Service's 1988 Love Stamp was lowered from the tower of the landmark building.

On New Year's Day in Boston, the L Street Brownies turned out for their 84th annual swim in the 40-degree water of Boston Harbor, some reminding the well-bundled onlookers that they take the plunge every day.

Not to be outdone, 100 members of the Newport, R.I., Polar Bears club jumped into the 36-degree Long Island Sound to raise money for a group that looks for missing children. At least 1,000 onlookers donated or pledged $3,000.

About a dozen cold-water lovers in Kenosha, Wis., made their annual foray into Lake Michigan, which was 30 degrees, and in Chicago, scuba-diving members of the 12-Fathom Dolphin Club took their annual plunge in the lake.

Several fatal fires marred the new year, including one in Los Angeles where firefighters were unable to save a woman and three children in a burning apartment even though they could hear scratching through a door that was secured by deadbolts.

New federal regulations limiting carry-on luggage went into effect with little fanfare. United spokesman Joe Hopkins said in Chicago that the new regulations only slightly modified the airline's practice.

Four spectators at the Rose Parade were injured slightly by a runaway horse, and a parade mechanic and a teen-age volunteer were seriously injured when they fell under the wheels of a float. Police arrested more than 450 people at the parade, most for public drunkeness.

Six college football bowl games were being played Friday.

There were other New Year's celebrations:

-Near Colorado Springs, Colo., 30 members of a climbers club reached the summit of Pike's Peak and set off a 20-minute fireworks extravaganza that could be seen in Denver, 60 miles to the north.

''It was terribly cold,'' said David Kosley, the club's newest member. ''We figured the wind chill to be between 65 and 80 below zero. The actual temperature was about 10 or 15 below.''

-In Philadelphia, about a dozen people gathered at the Betsy Ross House as officials laid an evergreen wreath with a red, white and blue bow at the tomb of the legendary flagmaker on the 236th anniversary of her birth.

-In Kansas, two Army lieutenants from Fort Riley jumped out of an airplane over the prairie at the stroke of midnight. The two, carrying flashlights visible for three miles, landed safely at the Wamego airport.

''It just seems like a good way to start a leap year,'' said Lt. Bryan Welch.