Thousands of Palestinians mounted a public protest against Israel's blockade on Gaza yesterday but the turnout fell short of the 40,000 to 50,000 that organisers had hoped would form a 25-mile human chain from north to south of the strip.

While ordinary Palestinians protested peacefully, militants launched several rockets at southern Israel, badly wounding a nine-year-old in the town of Sderot.

Jamal al-Khadary, of the People's Committee Against the Siege, which organised the protest, said neither the low turnout nor the rockets marred their message to the international community. "The important thing is to tell the world about what's happening in Gaza," he said.

The organisers were hoping to repeat Hamas's success last month when it buoyed community spirit and drew international attention to the human effect of Israel's isolation of Gaza by blasting open the wall on the border between the impoverished territory and Egypt.

Hundreds of thousands of Palestinians streamed across to buy food, medicines, building materials and other items in short supply as a result of the blockade.

Israel feared a similar breach would occur yesterday and put its military on high alert, deploying extra troops and police officers around Gaza's perimeter.

"Israel will work to avoid a deterioration of the situation but declares unequivocally that Hamas must assume full responsibility if that happens," Israel's foreign minister, Tzipi Livni, and its defence minister, Ehud Barak, said in a joint statement.

But yesterday's protest in Gaza was largely peaceful except for a group of young Palestinians who broke away at the end and hurled stones at the Israeli army, who fired their guns into the air in retaliation. The army detained 49 people.

Israel's air force also killed three Hamas militants and wounded at least four in two strikes on Gaza early yesterday before the protest began.

Israel began its economic siege last June in an attempt to isolate Hamas after it took control of Gaza. The militant Islamic group refuses to acknowledge the Jewish state's right to exist. Hamas and other militant groups have retaliated by launching rockets and mortar shells into Israeli suburbs.

The pressure of the blockade, which has plunged Gaza into poverty, has also caused anger among those who voted for Hamas in 2006 in the belief that it would clean up corruption and improve services.

Some analysts said yesterday's protest was partially an attempt at rebuilding confidence among Gazans.

"In their election promises to the people, it wasn't [about] jihad, it was about change and reform, meaning a better life, better services, more freedom of movement," said Shalom Harrari at the Institute for Counter Terrorism in Herzliya. "But on the contrary, life has become 100% worse because of internal factional fighting and the closure of the borders."

An International Crisis Group analyst, Nick Pelham, added that Hamas needed to "convince the population that their way was going to work" and as a result, it was "re-examining political and military options" for reopening Gaza.