EAST BERLIN (AP) _ Communist Premier Hans Modrow on Thursday defended his plan for a new internal security force and tried to quell growing dissent by indicating the opposition could have a part in his government.

Thousands of people formed a human chain around Parliament after Modrow's speech to legislators, and a non-Communist partner in the coalition government repeated threats to quit the Cabinet.

An independent trade union movement threatened to call a nationwide strike Jan. 25 if Modrow fails to prove the state secret police force has been dissolved.

In a nearly 90-minute speech before the national Parliament, Modrow defended his authority and appealed for national unity to fight the economic and political problems threatening his shaky government, which came to power after the fall pro-democracy movement.

''I cannot recall becoming premier through a coup,'' he said.

Modrow did not rule out the resignation of his transitional government for ''compelling reasons'' but said he expected it to last until nationwide elections planned May 6.

During the meeting, Parliament dismissed state security chief Lt. Gen. Wolfgang Schwanitz, whose agency the government promised last month to dissolve.

The hated secret police, popularly known as ''Stasi,'' used repressive measures to help ensure 40 years of Communist control.

Government officials say about 26,000 of the 85,000 state security employees have been taken off the payroll. Opposition groups want guarantees the force will be disarmed and assurances former agents will not have a part in a new agency.

''The aim is not to maintain the old structure'' of state security, Modrow said.

But he told Parliament ''there cannot be a security vacuum,'' and cited violence by neo-Nazi groups as well as ''terrorism, drugs and ecological crime.''

Modrow urged the opposition to make suggestions about how it would be prepared to ''participate directly and responsibly in the work of the government with competent personalities.''

However, Modrow did not say when or in what form such participation could take place.

He offered to have Parliament supervise the new security agency and dissolution of its predecessor.

Within hours, the Liberal Democrats, a non-Communist partner in the government, repeated their threat to leave.

''There can be no compromise with us'' on the secret police issue, said party spokesman Werner Fahlenkamp.

Two other non-Communist parties in government have threatened to quit over the issue.

Several thousand opposition supporters chanting and holding candles formed a human chain around the Parliament building. Demonstrators lowered several East German flags from their poles and ripped out the Communist symbol from the centers.

Representatives of the ''Initiative For Labor Union Reform,'' which claims 70,000 members, called for Modrow's resignation and the freezing of Communist Party assets.

They threatened to call a nationwide warning strike Jan. 25, to demonstrate their strength, if they are not convinced the security agency has been dissolved.

Earlier, East Berlin construction workers staged such a strike, which lasted one hour, and marched through the city center. The ADN news agency said 2,000 workers at a farm equipment factory in Erfurt also staged a strike Thursday.

Modrow, responding to the warning strikes, announced the government would suspend a plan for state aid to internal security employees who lost their jobs.

For the first time, opposition members sat as observers in Thursday's Parliament session. Modrow's speech was broadcast live on East German and West German TV.

News reports had said Modrow was considering calling for a national referendum to gauge the public support for his government before May, but he made no mention of this.

Several old guard Communists in the 500-seat Parliament lost their seats to younger deputies Thursday. Those ousted included Guenther Schabowski, a former Politburo member and close associate of disgraced former leader Erich Honecker.

At the start of the session, Parliament speaker Guenther Maleuda confirmed a West German report that Johanna Toepfer, once the No. 2 official in the government-run labor union federation FDGB, committed suicide Sunday.

Ms. Toepfer was forced to resign in November during the pro-democracy movement that drove out hard-line Communist leaders and cleared the way for scheduling of free elections May 6. Maleuda gave no details of the suicide.