Here’s a timeline of Saturday’s attacks and Israel’s retaliation.

The militant attacks across Israel on Saturday made for one of the most violent and deadly days of fighting in the region in decades.

Hamas, the militant group that controls the Gaza Strip, launched a series of rocket strikes that hit major cities across Israel, and it sent waves of fighters across the border into southern Israel, where they took over bases and seized hostages.

Here’s a timeline of what happened on Saturday and into Sunday in Israel and Gaza (times are local):

6:35 a.m. The first sirens warn of incoming rockets in central and southern Israel. This is the start of Hamas’s firing what will be thousands of rockets at Israel, striking even major cities like Tel Aviv and Jerusalem.

Muhammad Deif, the leader of the military wing of Hamas, said in a recorded message that the group had decided to launch what he called an “operation” so that “the enemy will understand that the time of their rampaging without accountability has ended.”

7:40 a.m. The Israel Defense Forces confirm that Hamas fighters have crossed from Gaza into southern Israel. The Israeli authorities ask residents of these towns, including Sderot, to hide in their homes.

8:15 a.m. The first sirens go off in Jerusalem, which rarely gets hit by rockets because of the Iron Dome defense system Israel uses. It was unclear if any rockets had struck the center of the contested city. But a witness said that rockets had landed in the wooded hills on the city’s western edge.

8:23 a.m. Nearly two hours after the first air raid sirens, the scale of the attacks is becoming clear. Hamas has taken hostages in southern Israel and fired wave after wave of rockets. Israel declares a state of alert for war, effectively calling up as many reservists as possible.

10:34 a.m. Israel says it has begun its war against Hamas.

10:46 a.m. Israeli fighter jets strike the first targets in Gaza.

11:35 a.m. Prime Minister Benjamin Netanyahu of Israel makes his first statement to the nation on the crisis, telling Israelis that they are at war.

12:21 p.m. The Israeli military sends troops to southern Israel to reclaim towns taken by Hamas militants. Israel estimates that Hamas has fired at least 2,200 rockets since the first sirens Saturday morning.

1:46 p.m. Israel says it has struck 21 buildings that house Hamas military operations.

2:29 p.m. The United States makes its first statement, with the National Security Council condemning the attacks and expressing support for Israel. Throughout the day, Antony J. Blinken, the secretary of state, will hold calls with leaders, including his counterparts across the Middle East, seeking support and laying out the U.S. position.

2:35 p.m. Mr. Netanyahu meets with his cabinet and releases a statement that they will “reinforce other frontiers,” an apparent reference to a possible threat from Hezbollah, the Islamist militant group that operates in southern Lebanon on the border with Israel.

6:08 p.m. President Biden speaks with Mr. Netanyahu to express his condolences and support. In a series of messages, including a speech, Mr. Biden says U.S. backing is “rock solid and unwavering.”

10:16 p.m. Mr. Netanyahu addresses the nation, calling Saturday a hard day and vowing to turn where Hamas operates into “cities of ruins.”

Overnight: Sirens continue to warn of rockets across Israel, with reports of some damage. Israeli fighter jets strike targets throughout Gaza.

2:19 a.m. The first phase of the war is near its end, with Israel claiming to have destroyed all the sites used to launch the attacks, Mr. Netanyahu posts on X, the social media site formerly known as Twitter. He writes that an “offensive formation” phase has started, without providing details.