Government officials would not confirm reports that the men, identified by sources in the Athens prosecutor's office as Albanians, were demanding 1 million euros ($1.33 million U.S.). They said their initial demand was for a driver to take them to the airport.
The men seized control of the bus at 5:50 a.m. about 10.5 miles east of the city center. The bus driver, a ticket inspector and a passenger escaped soon after, and 17 other hostages had been released about 13 hours into the standoff, officials said.
Police said six hostages remain inside.
As night began to fall, police pushed reporters away from the bus and tried to block their view of the scene.
One hostage told NET television by cell phone that the hijackers said they would release the women in return for a driver and set free the remaining passengers once they arrived at the airport.
Sources in the Athens prosecutor's office said both men were Albanians with criminal records in Greece. They speculated the men tried to hide their identities by pretending to be of a different nationality.
"They want a driver to take them to the airport. They want to go to Russia," said Stella Matara. "They don't want money or anything else. They have guns and dynamite. I don't know what kind. They are treating us well."
One hijacker also told a Greek television station he wanted to go to Russia.
Albanian Ambassador Bashkim Zeneli said police asked for his help because they thought the hijackers were Albanian. "I came here after I was told by Greek police that they are 99 percent sure the hijackers are Albanians," Zeneli told The Associated Press by telephone.
The hijackers have claimed to be Russian, but a Russian hostage reportedly tried to speak to them and they didn't seem to understand the language.
The hijackers were armed with at least one pump action shotgun, which they were seen firing out of a bus window at an approaching police car. They were also carrying a bag, but it was unclear whether they actually had explosives.
Outside, police — some carrying armored shields — crouched near the bus. Snipers were on nearby rooftops and a van was parked in front of the bus to prevent it from leaving. Negotiators were at the scene.
Premier Costas Caramanlis may delay a trip to a European Union summit in Brussels Thursday in order to deal with the crisis, his spokesman said.
The suspects boarded the bus at a stop in the suburb of Geraka just before dawn, firing warning shots through the roof, police said.
"Tell them to move the van from in front of us or we will blow up the bus," one of the hijackers said in Greek to Athens' Alpha television station. "Tell them to get all the police away from here. We want to go to the airport and fly to Russia. All passengers will get off there. We haven't harmed anyone, but if the driver is delayed, I said that I will strike."
Hundreds of thousands of immigrants live in Greece, including many from Albania and the former Soviet Union.
The hijacking was a first test for a Greek police force that underwent intensive training to deal with such situations during the Olympic Games. It was also the fifth time a bus has been hijacked since 1999.
The bus was on a route from the town of Marathon, east of Athens, to the city center. The bus stop was on a highway that was renovated for the Olympic Games and used for the marathon race, the 26.2-mile course from ancient Marathon to central Athens.