Israeli police have surrounded an encampment of thousands of Israelis opposed to the Gaza pullout.
Police appear unwilling to let the marchers go further than Kfar Maimon
Police forced back a surging crowd trying to march to the Gaza Strip, approximately 15km (10m) away.
Protesters and police traded punches, while several protesters were arrested. Three police officers were injured.
Police have been put on high alert as pro-settlement activists have vowed to press on with a march towards and into Gaza, which has been banned.
A massive security operation involving 20,000 police and troops has been launched to prevent up to 40,000 marchers from entering Gaza to disrupt next month's pullout.
Correspondents say the red-alert status reflects fears that events could spiral out of control.
The BBC's Nick Thorpe in Jerusalem says a month-long stand-off could develop, with activists trying different tactics to slip past police cordons.
Up to 40,000 activists have set up a makeshift camp near the farming village of Kfar Maimon, where they are expected to spend a second night in tents and sleeping bags.
They set up the camp on Monday evening after marching about 3km (1.8 miles) from their starting-point in Netivot.
The demonstrators say they are determined to push on towards the Gush Katif settlement bloc, via the Kissufim border crossing.
The Ynet website reported that 150 activists had broken through police lines and were heading towards the border.
Deputy Prime Minister Ehud Olmert said the plan to withdraw 21 Jewish settlements from Gaza had been approved by parliament and the demonstrators were trying to interfere with a legitimate decision of the Israeli government.
The Israeli army has sealed off Gaza from all non-resident Israelis since Thursday night.
Correspondents say the Israeli government's nightmare scenario is that more settlers will join the 8,500 already living in Gaza - making the withdrawal even more problematic than it already is.
Activists say they will stay until allowed to reach Gaza
After the pullout is completed, Israel will maintain control over Gaza's borders, coastline and airspace.
About 630 settlers will also be removed from four small West Bank settlements.
Israel has occupied Gaza and the West Bank since 1967. About 400,000 Israelis live in the territories, in settlements deemed as illegal under international law, although Israel disputes this.