Less than 24 hours after Turkey's new military ruler, Gen. Kenan Evren, pledged to curb the violence that has shaken this nation, leftist extremists warned they would retaliate against the military regime and within hours an Istanbul policeman was killed.

Meanwhile a left-wing extremist in Istanbul was killed in a shootout when friends tried to free him from police custody. In eastern Turkey, a man was killed in a clash with the Army that left two of his companions wounded and in military custody.

As if answering Evren's promise to bring peace to the nation, made in a press conference Tuesday, Dev-sol, Turkey's most organized Marxist underground group, telephoned newspapers in Istanbul this morning to warn that in the coming days it would "retaliate" against the military junta that assumed power here last Friday.

Hours after that call, a district police chief, Aykut Genc, was gunned down as he got into his car in the Sisli district of Istanbul to go to work. His assailants, who pumped more than 10 bullets into him, escaped.

Police authorities said later that the precision and style of the slaying were similar to that of former prime minister Neihat Erim, who was killed in Istanbul in July. Dev-sol claimed responsibility for Erim's killing.

Later as police were transporting a student leader, Zeki Yumurtaci, to another jail, they were ambushed by a group of terrorists seeking to free him.

Yumurtaci, a leader of a group known as the Marxist-Leninist Armed Propaganda Union, was killed in the shootout. Last year Yumurtaci's group claimed responsibility for the death of five Americans, including two servicemen, in two separate incidents in Istanbul.

The third violent incident of the day occurred in the eastern town of Urfa near the Syrian border, where a military sweep against suspected leftist strongholds resulted in another armed clash. One suspect was killed, two wounded and three captured.

The three deaths brought to five the number of persons killed in political violence since the Army takeover last Friday. Before the coup the average body count from terrorism around the country had risen to 25 persons a day, bringing the total deaths for the year to over 2,000 and providing the military with their main justification for taking over the country.

[Five cars belonging to members of the U.S. military mission in Athens were set on fire Tuesday night and a clandestine organization called the Revolutionary Left claimed responsibility, saying it was in protest for alleged U.S. support for the Turkish Army takeover, Reuter reported. There were no injuries.]

The military's ability to stop terrorism in Turkey will be its greatest test. Before the coup, 20 of the nation's 67 provinces had already been under martial law for the past two years.

The armed forces are intensifying their efforts to round up suspected terrorists around the country. Approximately 4,000 suspected extremists as well as 100 members of the dissolved parliament, including the leaders of the country's four major parties, have been detained so far.

The new military rulers today decreed that persons could be detained for 30 days without charges.

Military sources said that by the end of the month many of those arrested would be put on trial but so far they have given no indication that they would also try the politicians, as the military did after a coup in 1960.

Those trails resulted in the execution of prime minister Adnan Menderes and 14 of his closest associates.

Evren said Tuesday that the detention of the nation's political leaders was only a temporary measure and that when it was considered safe to do so, they would be freed from "protective detention."

Military sources, however, have indicated privately that the junta would seek to bar ousted prime minister Suleyman Demirel and opposition leader Bulent Ecevit from any future political role.

The same sources said that the militiary was also considering putting on trial two other key party leaders, Col. Arpaslan Turkes, the head of the neofascist National Action Party, and Necmettin Erbakau, the leader of the Moslem revivalist National Salvation Party, on charges of violating the constitution.