Update 2.36pm: Two Palestinians have been killed after Israeli troops opened fire on protesters across the border fence, health officials said.
The shootings came as thousands of Palestinians joined the fourth weekly protest on Gaza's border with Israel on Friday, with some burning tyres or flying kites which carried flaming rags.
Huge black plumes of smoke from the blazing tyres engulfed the area, as Israeli troops fired tear gas and live bullets, witnesses said.
Gaza's health ministry said 40 protesters were injured, but did not say how many of those were wounded by gunfire or overcome by tear gas.
The protests are part of what organisers, led by Gaza's ruling Hamas group, have billed as an escalating showdown with Israel, to culminate in a mass march on May 15. Organisers have made conflicting statements about whether they plan an eventual mass border breach.
In the past three weeks, 28 Palestinians were killed and hundreds wounded by Israeli troops firing from across the border fence.
In addition, two Palestinian men, aged 24 and 25, were shot and killed in a border area in northern Gaza, the health ministry said. The Israeli military had no immediate comment.
Hamas said the protests are aimed at breaking a crippling border blockade that was imposed by Israel and Egypt after the Islamic militant group overran Gaza in 2007, a year after winning Palestinian parliamentary elections.
The Israeli military has said it is defending Israel's border and that its troops, including snipers, only target "instigators". It has also accused Hamas of using mass protests as a cover for attacks.
Earlier: Palestinian man dies after being shot by Israeli soldier
A 25-year-old Palestinian man has died after being critically wounded by Israeli army fire near Gaza's border with Israel.
The Gaza health ministry said the man had been shot in the head.
It did not provide details about the circumstances of today's shooting, which came as Palestinians were gathering for a fourth weekly mass protest on the border.
The Israeli military said it is looking into the incident.
Before Friday, 28 Palestinian protesters had been killed and hundreds wounded by Israeli army fire from across the border fence since the protests began in late March.