In a decision that has triggered bitter divisiveness among liberals here, Massauchusetts Secretary of State Paul Guzzi yesterday entered a crowded Democratic senatorial race against embattled Sen. Edward N. Brooke (R-Mass).

Guzzi, a popular, progressive liberal who gained wide publicity with his vigorous crackdown against the fraudulent commodity options firm Lloyd, Carr and instantly became the Democratic front-runner by virtue of being the only candidate in the field of eight so far with statewide recognition.

Many liberals here, including former Guzzi campaign workers from his 1974 race have already declared their support for Paul Tsongas, also a progressive liberal considered a favorite until Guzzi entered the race.

"It will be a blood bath between friends, a civil war, and nobody wants that," said Democratic National Committee Woman Betty Taymor, a campaign worker for Guzzi in 1974 now working as a campaign coordinator for Tsongas in Guzzi's hometown.

However, political analyst John Gorman, who, along with presidential pollster Patrick Caddell, runs Cambridge Survey Research Inc., noted that both Gussi and Tsonga are "quite clearly in the mainstream of Democratic politics in Massachusetts," and downplayed the importance to the voters of the fight brewing among doctrinaire liberals.

In contrast to the Dixieland bands, balloons, crowds of supporters and other political fanfare at Guzzi's announcement, Howard Phillips - a former Republican who was placed in charge of dismantling the poverty program in President Nixon's administration - announced yesterday to a small, quiet gathering of supporters and reporters, that he, too, would run.

Phillips, founder of the Conservative Caucus, and the only avowed conservative in the race, described his mostly liberal opponents as "peas in a pod" . . . they are only in it because of Brooke's vulnerability. Phillips is running as a Democrat.

Guzzi, whose announcement falls just 13 days before the filing deadline, was quoted in early May as saying he would not run against Brooke because he is the nation's only black senator. The statement, however, came before the first published reports that Brooke had lied in sworn statements in connection with his divorce.

Circumstancts have changed," he told reporters yesterday. "It is very unclear to me as to who the republican candidate will be for U.S. Senate. I have serious doubts now that Ed Brooke will be the Republican candidate."

Meanwhile, the U.S. ambassador to the Law of the Sea Conference former attorney general Elliot L. Richardson, reported to be under strong Republican political pressure to enter the race, has said he will not declare his candidacy unless Brooke drops out.

Boston Democratic Mayor Kevin White said he is also considering entering the Democratic senatorial primary.

However, with a looming filing deadline, the possibility of obtaining 10,000 signatures to enter the race is becoming increasingly difficult.