* Attack comes hours before Greece taps bond markets
* No immediate claim of responsibility
* Police suspect leftist or anarchist group (Updates with details, government spokesman quote)
By Karolina Tagaris and Renee Maltezou
ATHENS, April 10 (Reuters) - A car bomb exploded outside a central bank building in Athens early on Thursday, smashing windows but causing no injuries, just hours before Greece was due to make its first foray into the international bond markets in four years.
The dawn blast, which police believe was carried out by leftist or anarchist guerrilla groups, also came a day before a planned visit to Athens by German Chancellor Angela Merkel.
An anonymous caller warned a newspaper about 45 minutes before the explosion just before 6 a.m. (0300 GMT), saying it contained about 70 kg (155 pounds) of explosives, a police official said on condition of anonymity. A news website also received a warning call.
Witnesses saw debris strewn across the street in one of the busiest parts of central Athens that is lined with banks, shops and a mall. A second police official, who also declined to be named, said the force had yet to determine the amount and type of explosives used in the attack.
"It is clear that the attackers are trying to set the agenda," Government Spokesman Simos Kedikoglou told Skai TV. "We will not allow the terrorists to succeed."
Makeshift bomb and arson attacks have escalated since Greece adopted unpopular austerity measures in exchange for multi-billion euro bailouts by the European Union and International Monetary Fund from 2010.
Athens, which is seeking to send a strong political and economic signal that it is finally exiting its debt crisis, is expected to tap bond markets later on Thursday.
But public anger remains high after six years of recession that has sent unemployment to record highs of nearly 28 percent, eroded living standards and shut down thousands of businesses.
Greeks have had their incomes slashed by almost a third since 2010 and strikes against austerity are frequent.
Attacks against banks, government buildings, politicians, journalists and businesspeople are fairly common in Greece, which has a long history of political violence.
In 2009, a powerful car bomb exploded at the Athens stock exchange, slightly injuring one person and damaging the building One of Greece's most militant guerrilla groups, Revolutionary Struggle, claimed responsibility for that attack. (Additional reporting by Harry Papachristou and Alkis Konstantinidis; Editing by Gareth Jones)