Why did Hamas attack Israel now?
Hamas spokesperson Khaled Qadomi has told Al Jazeera that the group's military operation is in response to all the atrocities the Palestinians have faced over the decades.
"We want the international community to stop atrocities in Gaza, against Palestinian people, our holy sites like Al-Aqsa. All these things are the reason behind starting this battle," he said.
"This is the day of the greatest battle to end the last occupation on Earth," Mohammed Deif, the Hamas military commander said, adding that 5,000 rockets were launched.
"Everyone who has a gun should take it out. The time has come," Deif said, according to reports.
Hamas called on "the resistance fighters in the West Bank" as well as "our Arab and Islamic nations" to join the battle, in a statement posted on Telegram, reports Al Jazeera.
The rockets were fired as far north as Tel Aviv. Hamas also sent fighters into southern Israel.
Israeli media said that gunmen had opened fire on passers-by in the town of Sderot, and footage circulating on social media appeared to show clashes in city streets as well as gunmen in jeeps roaming the countryside. One report claimed Hamas fighters had taken control of multiple Israeli civilian population centres, where residents were begging for help from their government.
The Israeli army says dozens of fighter jets were carrying out attacks on Hamas targets in the Gaza Strip.
Currently, the gun battles are taking place in and around the towns of Kfar Aza, Sderot, Sufa, Nahal Oz, Magen, Be'eri, and the Re'im military base, the Times of Israel reported.
The Israeli army warned Israelis who live near Gaza to stay in their homes or head to shelters.
Israeli Prime Minister Benjamin Netanyahu says his country is in a war it "will win".
Israeli Defence Minister Yoav Gallant said Israel "will win this war." Hamas, he said, had made a "grave mistake this morning and launched a war against the State of Israel".
Israeli media reported gunbattles between bands of Palestinian fighters and security forces in towns in southern Israel. Israel's police chief said there were "21 active scenes" in southern Israel
Al Jazeera's Walid al-Omari, bureau chief in Ramallah, says Israeli forces surrounded a house in the Ofakim settlement and negotiations are under way with Palestinian fighters who are allegedly holding hostages.
The Ministry of Health in Gaza has issued a call for blood donations at hospitals in the besieged enclave.