The press conference was called after the West African nation's parliament voted for the new constitution on Monday, changing the presidential system to a parliamentary one.
With less than a month before legislative elections, the opposition say they fear the change will clear the way for long-serving President Faure Gnassingbe to remain in power indefinitely.
Around 30 police personnel armed with truncheons broke up the press conference put together by opposition parties and civil society groups, saying the event in the capital, Lome, did not have the necessary authorization.
"It's unacceptable, the Togolese regime is allowing itself absolutely everything, having changed the constitution on the sly," organizer and head of the Parti des Togolais (PT) Nathaniel Olympio told AFP.
"By depriving the Togolese people of the right to choose their president through this constitutional coup, the regime has crossed a red line," Olympio said in a statement after the event at the headquarters of the ADDI opposition party.
"I call on the Togolese people to stand up and stop this disastrous plan," he said.
The change to the constitution, proposed by a group of lawmakers mostly from the Union for the Republic (UNIR) ruling party, was adopted almost unanimously on Monday.
The president will be chosen "without debate" by lawmakers "for a single six-year term," and not by the public, according to the new text.
Currently, the president can serve a maximum of two five-year terms.
The country's opposition, which boycotted the last legislative elections in 2018 and condemned "irregularities" in the electoral census.
FILE - Republic of Togo's President Faure Essozimna Gnassingbe Eyadema gestures as he is welcomed by French President before a working lunch at the Elysee Presidential Palace in Paris on April 9, 2021.
It is not yet known when the change — which was approved with 89 votes in favor, one against and one abstention — will come into force.
In 2019, members of parliament revised the constitution to limit presidential terms to two, but it did not apply retrospectively, leaving President Faure Gnassingbe free to stand for the next two elections.
Gnassingbe — who’s been in power since 2005 — succeeded his father General Gnassingbe Eyadema, who seized power in a coup more than 50 years ago.