Tens of thousands of angry Turks marched through the streets of Istanbul this afternoon, protesting measures reportedly taken by Bulgaria against the ethnic Turkish minority living there.

Western press reports in recent months have described systematic efforts by the Bulgarian authorities to force ethnic Turks to adopt Slavic names and drop their Moslem religion. A recent report by the British Broadcasting Corp. mentioned an armed clash between the Bulgarian military and members of the minority resisting the measures.

The Bulgarian authorities have denied the charges, contending that ethnic Turks who adopt Slavic names do so voluntarily.

Turkish Prime Minister Turgut Ozal has urged high-level discussions with Bulgaria on the problem. At a press conference earlier this month, he offered to open Turkish borders to allow the mass migration of Turks from Bulgaria. Similar mass migrations have taken place twice in the past three decades -- in the 1950s and 1960s. So far, the Bulgarians have refused any dialogue.

By some estimates, Bulgaria's ethnic Turks number about 1 million and are the country's largest minority, representing about 10 percent of the population. Bulgaria was ruled by the Ottoman Turks for nearly 500 years -- from the end of the 14th century until a revolt in 1876. Bulgaria established an independent kingdom in 1908.

Most ethnic Turks remaining in Bulgaria today live in areas adjoining the Black Sea and the Turkish border.

Turkish public sentiment, aroused by news coverage of the dispute, has run high enough that authorities had no choice but to allow today's march, the first public demonstration of any sort in Turkey since Sept. 12, 1980, when the military staged a coup and banned all demonstrations.

About 40,000 persons gathered early this morning on the shores of the Marmara Sea carrying banners and Turkish flags. The marchers headed toward the main square around noon, chanting slogans condemning Bulgaria's Communist government and denouncing Bulgarian leader Todor Zhivkov.

Steel-helmeted troops and police officers took heavy security measures throughout the march, which culminated at a square near the mayor's office.

One immigrant, who refused to identify himself in order to protect his family still living in Bulgaria, said, "Turkey tried to solve the problem with the best of intentions through bilateral talks. The only thing to be done now is to activate international public opinion.

"A great part of Bulgaria's income comes from the Middle East. An economic embargo, especially by the Islamic countries, could be effective, and Bulgaria might retract its steps."

There seems to be little that Turkey can do about the matter other than to rely on world opinion to bring pressure on the Bulgarians. Today's demonstration, although largely symbolic, was an emotional outlet for Turks, who feel their hands tied in the dispute.