Egypt's largest opposition grouping on Tuesday dismissed a call by the country's president for national reconciliation talks, saying it was "too late" for such talks ahead of planned mass protests at the end of the month to demand Mohammed Morsi's ouster and early presidential elections.

President Morsi made the call during a fiery speech Monday over Ethiopia's plans to build a dam on the Blue Nile, a project Cairo claims would jeopardize the flow of the Nile River through Egypt and cause a critical water shortage in the country.

In the speech, Morsi urged Egyptians to unite behind a common stand, saying he was "ready to meet anyone to serve the nation's interest" and consolidate the country's internal front in the face of outside dangers.

"I call upon everyone to forget partisan conflict ... this is the time for real lining up," Morsi said, adding that "the nation forces us to be one line."

"I will not give up calling for a comprehensive national reconciliation," he said.

Critics accuse Morsi of using the Nile dam issue to whip up nationalist fervor and undercut the opposition's push for his ouster.

"Such a call is simply lip service on Morsi's part and tasteless PR," said Khaled Dawoud, spokesman of the National Salvation Front, the main opposition coalition.

"It is rather too late after Morsi failed to hold a single serious dialogue in his year in office," Dawound said.

Tensions are rising ahead of June 30, when Morsi marks one year in power as Egypt's first freely elected president following the 2011 uprising that toppled his predecessor, autocrat Hosni Mubarak.

The opposition has called for mass demonstrations to mark the anniversary by calling for his ouster.

Liberal and secular-minded groups accuse Morsi and his Muslim Brotherhood of causing deep polarization among Egyptians, saying he has not fulfilled his promises of creating an inclusive political process and instead has been enabling the Islamists to monopolize power.

The Brotherhood in turn charges the opposition of trying to unseat Morsi through street violence, instead of through the ballot box, and insists the opposition lacks grass-root support.

The Ethiopian dam issue has added to the overall discontent among Egyptians over the Islamist government's handling of the country's political, economic and security crises that have worsened since Mubarak's overthrow — first under transitional military rule, then following Morsi's election and year in office.

The $4.2 billion hydroelectric dam, which would be Africa's largest, challenges a colonial-era agreement that gave Egypt and Sudan the lion's share of rights to Nile water. Experts estimate that Egypt could lose as much as 20 percent of its Nile water in the three to five years needed for Ethiopia to fill a massive reservoir.

In Monday's speech, Morsi said Egypt was not calling for war but is willing to confront any threats to its water security.

"If it loses one drop, our blood is the alternative," he said to a raucous crowd of largely Islamist supporters.