MOSCOW--Communist Party leaders added an unexpected third day to their pivotal meeting yesterday and sent President Mikhail S. Gorbachev back to the drawing board to fill in holes in his blueprint for ending the party's monopoly on power.

The extension of the party session, which was supposed to end yesterday, was a clear sign of the controversy generated by Gorbachev's proposals to revamp the country's political structure.

Central Committee sources said most speakers agreed with Gorbachev's proposal that the party's monopoly on power, enshrined in the Soviet constitution, must end.

"All unanimously think it has become obsolete. It has no meaning," Central Committee member Vladimir P. Anishchev told reporters who gathered outside the Kremlin's Spasky Gate to await news. The session was closed to journalists.

However, partial transcripts of two days of the session, in which 51 speakers took the floor, indicated that neither radical reformers nor hard-liners were satisfied with Gorbachev's proposal.

Delegates said last night's Central Committee session was suspended so a commission, headed by Gorbachev, could complete changes to his nearly 20-page platform. A Central Committee source quoted Gorbachev as saying that the 60-member commission was only half-finished, even though it met all night Monday.

He said the meeting would resume this morning.

Delegates interviewed in the last two days indicated a common objection: the platform contains few specifics.

"A lot of its points, and this is what has been said by many speakers, need major changes and reinforcement, especially in the direction of stepping up decisive action," Leonid A. Bibin, a non-voting Central Committee member, said in a Soviet TV interview yesterday.

Bibin said he and others want the platform to stress that the party must remain united.

Gorbachev has placed himself in the middletrying to forge compromises.

Still, the Soviet leader's platform has yieldedto the demands of political reformers--includingthe thousands of people who gathered near theKremlin wall Sunday--by removing the party'smonopoly and advancing to this summer a partycongress that will be empowered to clear outconservatives on the Central Committee.

Formally, the Central Committee has the powerto replace the party leader.

Indrek Toome, premier of the Baltic republic ofEstonia and a guest at the party meeting,emphasized the threat from hard-liners.

"I am worried about the wish of a fairly largeproportion of the people in this hall to reversethings, so as to clamp down on things and returnto the old order," Toome told Soviet TV.