Two Ukrainian men were being held after a suspected campaign of three bomb attacks in a month against mosques in the West Midlands which saw MI5 join the hunt for the explosives-makers.

Police and disposal teams were searching for more bomb-making material on Friday, a day after it emerged that the remains of an exploded bomb and debris had been found at the site of the Wolverhampton Central Mosque.

Officers were also trying to establish whether there was any connection to far-right violent extremism.

The attempted attacks all occurred on a Friday, usually a day when mosques would be busy. Police in the region had feared earlier this week that another attack would be staged on Friday, the Guardian has learned.

The concern caused by the bombs reflects rising tensions in the West Midlands which are also being stoked by a planned protest by the English Defence League in Birmingham city centre on Saturday, when about 1,500 supporters may attend. Police plan to deploy 1,000 officers to deal with the protest and a counter-march by anti-fascists.

The two men were arrested on Thursday and searches of their home and work addresses followed in Small Heath, Birmingham, which has a sizeable Muslim community.

On 21 June attempts were made to bomb mosques in Walsall, and a nail bomb was found outside a mosque in Tipton on 12 July. The newly discovered Wolverhampton bomb is thought to have exploded between the two dates on 28 June, just after the morning rush hour, after having been planted the night before.

The chief constable of West Midlands police, Chris Sims, took the "extraordinary" step earlier this week of ordering officers to work extended 12-hour shifts as counter-terrorism investigators tried to catch the alleged perpetrators.

During the investigation police concluded that there was information to trigger suspicions a device may have been placed near the Wolverhampton mosque some weeks before. That proved to be correct. The same source said other bomb-related material might be present at an address which continues to be searched.

Counter-terrorism detectives were continuing to question the two Ukrainian men, aged 22 and 25, arrested on Thursday in connection with the suspected attempted attacks in Tipton and Walsall. The pair were being held on suspicion of being involved in the commission, preparation or instigation of an act of terrorism.

West Midlands counter-terrorism police also searched a software company where the two men were believed to have been working on a university placement.

Clive Martell, chief executive of the software company Delcam, provided a few further details about the arrested men: "The two men were on work placements with us, but were not employees of the business. They are studying at a foreign university and are engineering students."

Martell told the Birmingham Mail that the two were studying in an eastern European country: "One of the men has been with us for four months and the other has been with us for two months.

"They were around halfway through the course and we understand that they met each other whilst on the placement."

West Midlands police said specialist officers called in to search the area around the mosque on Thursday had found the "seat of an explosion" and debris on a nearby roundabout at Wolverhampton. An area around the mosque was sealed off at 8pm on Thursday as a military bomb disposal team was deployed.

Assistant Chief Constable Marcus Beale, who is in charge of the investigation, said: "Officers searching the area [around the Wolverhampton mosque] have found the seat of an explosion and debris on the island near the mosque."The investigation is being led by specialist officers and staff from our counter-terrorism unit who are being supported by a range of departments from across the force.

"We recognise the impact news of the latest find will have on the communities of Wolverhampton and further afield. We're working hard to complete our inquiries so that the area can be returned to normality.

"While the investigation is in its very early stages, early indications are that the explosion happened on Friday 28 June."

Iftikhar Ahmed, representing Wolverhampton Central Mosque, which reopened for prayers on Friday, said there were anxieties in the area but they would be allayed. He said: "Of course there are concerns in the community but we have faith in the police and intelligence services that the incidents are being dealt with.

"They already have two suspects in custody and for our community today it is work as normal."

The Friday before, residents were evacuated after a blast near the Kanzul Iman Masjid (mosque) in Tipton left nails and debris scattered outside. Police believe the intention was to kill or maim after the attack in broad daylight. No one was injured but some minor damage to property was reported.

A small component of the suspected nail bomb was found in a garden nearby and specialist army bomb disposal officers were called to the scene to carry out a controlled explosion.

This came after more than 100 residents were evacuated from their homes in the Caldmore area of Walsall last month after an explosion near the Aisha Mosque and Islamic Centre. No one was hurt in the blast, which caused minimal damage to a wall near the mosque.

Assistant Chief Constable Sharon Rowe said: "At the direction of the Chief Constable and with immediate effect, all police officers are to work 12 hour shifts.

"This extraordinary requirement is to support a large scale policing operation in place across the region as the force manages two high-profile counter terrorism investigations and moves towards a weekend with a significant protest expected in Birmingham City Centre."The communities secretary, Eric Pickles, said: "The calm and measured community response to yesterday's discovery of a very real terrorist threat to worshippers at Wolverhampton Central Mosque showed a city united against hate.

"The fact that Friday prayers went ahead today with hundreds of worshippers in attendance is proof yet again that those who seek to sow the seeds of division have failed."