Netanyahu, 75, is Israel's first sitting prime minister to be charged with a crime.
He is testifying at the same time Israel is engaged in a war in the Gaza Strip and facing possible new threats posed by regional turmoil, including in Syria.
Last week judges ruled that Netanyahu - indicted in 2019 - must testify three times a week, forcing the longtime Israeli leader to juggle between the courtroom and the war room at Israel's Defence Ministry, minutes away from the courthouse.
He took the stand for about four hours and will resume testifying on Wednesday.
Twice his military secretary handed him written messages, the first time requiring a recess and underscoring his having to do double duty as prime minister.
The leader of the conservative Likud party, Netanyahu assailed the Israeli media for what he called its leftist stance and accused journalists of having hounded him for years because his policies did not align with a push for a Palestinian state.
"I have been waiting for eight years for this moment to tell the truth," Netanyahu told the three-judge court.
"But I am also a prime minister ... I am leading the country through a seven-front war. And I think the two can be done in parallel."
Netanyahu was indicted in three cases involving gifts from millionaire friends and for allegedly seeking regulatory favours for media tycoons in return for favourable news coverage.
He denies any wrongdoing and has pleaded not guilty.
"Had I wanted good coverage all I would have had to have done would be to signal toward a two-state solution ... Had I moved two steps to the left I would have been hailed," he said.
He stood rather than sat in the witness box while testifying.
In lengthy replies, he portrayed himself as a staunch defender of Israel's security, withstanding pressures from international powers and a hostile domestic media.
Netanyahu smiled confidently when he entered the Tel Aviv District Court about 10am.
The trial was moved from Jerusalem for undisclosed security reasons and convened in an underground courtroom.
Before Netanyahu took the stand, his lawyer Amit Hadad laid out for the judges what the defence maintains are fundamental flaws in the investigation.
Prosecutors, Hadad said, "weren't investigating a crime, they were going after a person".
A few dozen protesters gathered outdoors, some of them supporters and others demanding Netanyahu do more to negotiate the release of about 100 hostages still held by Hamas in the Gaza Strip.
Israel has been waging war in the enclave against the Palestinian militant group for more than a year, during which Netanyahu had been granted a delay for the start of his court appearances.
Last Thursday, judges ruled that he must start testifying.
In the run-up to his court date, Netanyahu revived familiar pre-war rhetoric against law enforcement, describing investigations against him as a witch hunt.
Before the war, Netanyahu's legal troubles bitterly divided Israelis and shook Israeli politics through five rounds of elections.
His government's bid last year to curb the powers of the judiciary further polarised Israelis.
The shock Hamas attack on Israel on October 7, 2023 and the ensuing Gaza war swept Netanyahu's trial off the public agenda as Israelis came together in grief and trauma.
But as the war dragged on, political unity crumbled.
In recent weeks, while fighting abated on one front after Israel reached a ceasefire with Hamas' Lebanese ally Hezbollah, members of Netanyahu's cabinet - including his justice and police ministers - have clashed with the judiciary.
His domestic legal woes were compounded last month when the International Criminal Court issued an arrest warrant for him and his former defence chief Yoav Gallant along with a Hamas leader, for alleged war crimes in the Gaza conflict.
