Egyptian police have blocked an opposition convoy carrying medical supplies to the Rafah border crossing with the Gaza Strip.
Abdel Fatah Rizq, who helped co-ordinate the convoy, said police had arrested 50 activists on Monday when they tried to gather at the Journalists' Syndicate in downtown Cairo.
The convoy was preparing for a 300km journey across the Sinai desert to reach the Palestinian border.
Conflicting reports from a security official, who chose to speak on condition of anonymity, said that at least 20 protesters were arrested after they surrounded campaigners where the convoy was due to depart.
"Activists from all over the provinces were planning on joining the convoy on its way through the Sinai to Rafah crossing,'' Abdel Rizq said.
Police said three members of the Muslim Brotherhood, Egypt's main opposition group, were arrested at the Suez Canal on Sunday night while trying to reach the border.
Another 14 activists from Kefaya, another Egyptian opposition movement, had been arrested in Rafah, also on Sunday, while staging a protest.
Many Palestinians living in Gaza depend on smuggled goods siphoned through underground passages between Egypt and Gaza.
Israel has maintained that the tunnels are used to smuggle arms and explosives, having sealed off the coastal strip after Hamas seized power in Gaza in June 2007.
Campaigners included judges, independent members of parliament, Muslim Brotherhood members and other party figures who want to protest the continued closure of the Rafah crossing by Israel and Egypt.
Police have set up checkpoints on the road between Ismailiya and the town of El Arish in northern Sinai, at least 45km from Rafah where only residents are allowed to enter, a security official said.
Last month, police blocked a similar convoy headed to the Rafah crossing, Gaza's only exit to the outside world that is not under the control of Israel.
Sympathy for the Palestinians blockaded in Gaza by Egypt and Israel runs high among Egyptians.
Egypt occasionally permits people to enter and leave Gaza, but has refused to open the Rafah crossing permanently.
Hosni Mubarak, the Egyptian president, said on Monday that Egypt would not reopen the crossing as long as Hamas controls Gaza.
State media reported Mubarak as saying: "We are still committed to the 2005 agreement," referring to the agreement under which the Palestinan Authority, Israel and EU monitors supervise the Rafah crossing.
Source: Al Jazeera and agencies