Democratic Gov. Brendan T. Byrne took an early lead in New Jersey's gubernatorial election last night and appeared headed toward an astounding political comeback.

Shortly after the polls closed at 8 p.m., computer projections by three New York City television stations declared Byrne a winner with 54 per cent of the vote.

If he sustained his lead, Byrne would have written a new chapter in the manual of come-from-behind campaigning. Seventy per cent of the voters in the Democratic primary voted against him, but he emerged from a 10-candidate field with just enough votes to win renomination.

Trailing Byrne with 46 per cent of the projected vote was the Republican aspirant, Raymond H. Bateman, who was held up as the party's hope for reversing the post-Watergate tide in the state.

Torential rain blanketed the Northeast for most of the day, resulting in lighter than expected voter turnouts in New York and vote-heavy northern New Jersey.

Republicans predictably interpreted the downpour and the light voter turnout as a good omen for Bateman and for New York State Sen. Roy Goodman, both of whom had been waging uphill battles against their Democratic opponents to win control, respectively, of the State House in Trenton and City Hall in Manhattan.

Just as predictably, Democrats said the flooded streets and diminished vote totals would have no measurable impact on their candidates.

Democrat Edward Koch, a five-term congressman from Manhattan, was expected to breeze to victory and become New York City's 105th mayor.

In two primary races, Koch scored upsets over such formidable opponents as former Rep. Bella Abzug, Mayor Abraham D. Beame and Secretary of State Mario Cuomo, who was backed by Gov. Hugh L. Carey, Cuomo ran the general election on the Liberal Party line.

In New Jersey, pre-election polls pointed to a victory for Byrne in an election that was as much a referendum on the state income tax as anything else.

Central to the race was whether the unpopular new tax would be extended when it expires June 30.

Bateman said he would scrap the tax and institute budget cutting, improve the state's business climate and, if necessary, increase the sales tax by one cent.

Byrne, a staunch advotage of the income tax amid a storm of opposition, told the voters that Bateman's campaign promises would bring higher property taxes and reduce state aid to education.

A re-election victory for Byrne would be an extraordinary chapter in the manual of come-from-behind campaigning. Seventy per cent of the voters in the Democratic primary voted against him, but he still emerged from the 10-candidate field with enough votes to win renomination.

In doing so, Byrne left his party in disarray and entered the general election with an all-time personal popularity low.

He hired media wizard and political adviser David Garth to help manage his campaign, and methodically convinced New Jersey voters that they could vote for Bredon Byrne, even if they didn't like him.

Persistently, Byrne labeled Bateman a "sham" and a "fake" and his message appeared to get through. A final poll by Rutgers University's Eagleton Institute showed he had gained a dramatic turnabout to hold an eight-point advantage over Bateman.

The miserable weather in New York and New Jersey extended across the country but failed to dampen voter turnout elsewhere in the nation where mayors elected in 38 cities with populations over 100,000.

In some cities, black candidates who won dramatic primary victories tried to wrest control of municipal governments by bridging racial voting patterns established for years.

In a few cities, incumbents fought to hold their power, such as in Detroit, Minneapolis, Pittsburgh, Albony, N.Y., and Hartford, Conn.

Across the breadth of America, there were the promises of innovations and anomalies once the votes had been tallied.

Detroit became the first major U.S. city to match a black against another black in a mayoral general election.

Incumbent Coleman A. Young, elected the city's first black mayor in 1973, was heavily favored to beat Councilman Ernest C. Broune Jr. to retain control of the nation's fifth largest city.

Cleveland voters were assured of electing the youngest big-city mayor in the United States, no matter who won.

Republican Mayor Ralph Perk was defeated in Cleveland's September open primary, and he will be replaced by either of two young Democrats, state Sen. Edward Feighan, 30, or Municipal Court Clerk Dennis Kucinich, 31.

In heavily Italian-Slavic blu-collar Buffalo, Arthur O. Eve, a militant civil rights leader, was a strong favorite over his Republican and Conservative Party opponents to become New York state's first black mayor.

For the most part, the elections held little promise for revealing significant political trends, although Republican National Committee officials said yesterday they were closely watching the Louisville, Ky., election, in which voters decided the political future of Jefferson County Judge Todd Hollenbach in a do-or-die race against Republican challenger Mitch McConnell, a 35-year-old former Justice Department lawyer.

Although the race involved no more than the equivalent of county executive in the Louisville area, it captured the attention of politicians across the state who view the metropolitan area's 200,000 voters as the key to statewide party control.

In Kentucky, a Fiscal Court - the governing body over county commissioners - supersedes the power of mayors and other high elected officials. The chief executive is title a judge, although he does not normally reside over judicial matters.

Republicans hold none of the top city or county offices in Kentucky now, and national party officials said a victory by McConnell could spell new hope for Republicans in the 1978 congressional elections.