The arrests happened over the last two days in Niger's Diffa region, which borders Nigeria. Those taken into custody include Kaka Bunu, who police spokesman Adil Doro said was "involved in the recruitment of (Boko Haram) members."
Some of the suspects fled south, only to be arrested while on the run or in "their hiding places," said Yakubu Sumana Gawo, the governor of Niger's Diffa region.
Members of the Nigerien police's anti-terrorism unit are interrogating the suspects, all of them Niger citizens.
Cameroon, meanwhile, is holding hundreds of suspected Boko Haram militants in prisons in its Far North Region.
Two Cameroon security sources said more than 1,000 alleged terrorists are in custody in the central prison in the town of Maroua. Col. Joseph Nouma, commander of Cameroon's special military operation targeting terrorists, offered a more conservative number of more than 600 detained.
Nouma said his forces squared off Monday with Boko Haram fighters and beat them decisively.
The battle in Gnam-Gnam, which is 4 kilometers (2.5 miles) from the Nigerian border, began when Cameroonian soldiers on a reconnaissance mission "were ambushed by the terrorists," the Cameroonian colonel said.
It ended a few hours later with at least 100 militants and five Cameroonian soldiers dead, according to Nouma. He said Cameroonian troops also seized an armored vehicle, two machine guns and a large amount of ammunition.
Nigeria joined in fight by Niger, Cameroon
Even with these mass arrests and reported military victories, though, no one is under the illusion that Boko Haram is finished.
The group, whose name translates as "Western education is sin," has been waging a yearslong campaign of terror aimed at instituting its extreme version of Sharia law. Boko Haram's tactics have intensified in recent years, from battling government soldiers to acts disproportionately affecting civilians -- such as raids on villages, mass kidnappings, assassinations, market bombings and attacks on churches and unaffiliated mosques.
Much of this violence has taken place in Nigeria. But neighboring countries, such as Cameroon and Chad, have also been hit increasingly hard.
Niger became more of a target following its decision to join the regional alliance fighting Boko Haram. The group has been blamed for raids and suicide bombings in recent days in Diffa, which is in the far southeastern part of the country.
In response, Niger's government last Wednesday declared a state of emergency in the region that gives authorities the OK to go house to house in its crackdown on Boko Haram.
Cameroon has been battling the terrorist group for longer.
The African nation has deployed about 7,000 troops across the Far North Region, where Boko Haram has scaled up attacks over the past year.
These include cross-border raids and attempts to take over a Cameroon military base in Kolofata and another in Achigashia, forcing Cameroonian soldiers to retreat and then for the first time use airstrikes against the insurgents.
And last month, Boko Haram militants kidnapped 80 people
in northern Cameroon, Said Abdulkarim, a journalist for the state broadcaster, told CNN.