RAMALLAH, West Bank, July 20 (Reuters) - Jailed leader Marwan Barghouti has called on Palestinians to stage mass rallies in September in support of a diplomatic bid to gain U.N. membership for a state of Palestine.

Barghouti, a figure widely respected among many Palestinians, said taking the statehood quest to the United Nations was part of a new strategy that would open the door to "peaceful, popular resistance".

"I call on our people in the homeland and in the diaspora to go out in a peaceful, million-man march during the week of voting in the United Nations in September," Barghouti said in a statement written from his jail cell in Israel.

With the Middle East peace process at a standstill, the Palestinians, backed by the Arab League, have decided to seek full admission to the United Nations as part of what they are describing as a new approach to their national struggle.

Israel is wary that the September bid could serve as a trigger for protests inspired by uprisings across the region.

Speaking on condition of anonymity, an Israeli military commander said on Tuesday Israel would reinforce its border defences in anticipation of such protests.

The United States, the main sponsor of the two-decade-old peace process, has objected to the Palestinian diplomatic offensive, instead calling for a resumption of negotiations that were derailed by an impasse over Jewish settlement expansion. Israel says the Palestinians aim to isolate and delegitimise it.

A U.S. veto at the Security Council is likely to thwart the Palestinian bid for full U.N. membership for a state of Palestine in the West Bank and the Gaza Strip with East Jerusalem as its capital.

"ADVANCING OUR CAUSE"

In such a scenario, the Palestinians have indicated they will table a U.N. General Assembly resolution that would elevate their status to that of "a non-member state" from an "observer".

They expect such a move to succeed, thanks to the support of 120 countries -- a number they expect to grow to at least 130.

Barghouti, a leading member of the Fatah movement led by President Mahmoud Abbas, called on all Palestinian parties to back the diplomatic offensive and "to confront the American-Zionist veto".

For many Palestinians, Barghouti remains a symbol of their national struggle. His supporters portray him as a Palestinian Nelson Mandela -- a charismatic figure who could unite Palestinians and galvanise their quest for statehood.

He was convicted of murder for his role in attacks on Israelis in the last Palestinian uprising, or Intifada, and sentenced to life in jail by Israel in 2004.

Riyad Mansour, the Palestinian envoy to the United Nations, said an upgrade of the Palestinians' U.N. status to "non-member state" would bring with it benefits including allowing the Palestinians full membership of U.N. agencies.

"You are automatically allowed to become a full member of all the agencies: WHO, UNESCO, UNICEF, etc," he told journalists during a briefing in Ramallah on Wednesday.

He said September's U.N. General Assembly would mark a historic moment in the Palestinian struggle. "We are going to accomplish certain things in this coming session that will be extremely significant to advancing our cause," he said. (Editing by Alistair Lyon)